Magnetic powder, method for producing the same and magnetic recording medium comprising the same

ABSTRACT

A magnetic powder consisting of substantially spherical or ellipsoidal particles comprising a transition metal which comprises iron and a rear earth element which is mainly present in the outer layer of the magnetic powder particles, and having a particle size of 5 to 200 nm, a coercive force of 80 to 400 kA/m and a saturation magnetization of 10 to 25 uWb/g.

This nonprovisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 2002-274435 filed in JAPAN on Sep. 20,2002, which is herein incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a magnetic powder comprising iron and arare earth element, a method for producing the same and a magneticrecording medium comprising the same. In particular, the presentinvention relates to a magnetic recording medium which is particularlysuitable for use in ultra-high density recording, for example, a digitalvideo tape, a backup tape of a computer, a large capacity Floppy® disc,etc.

BACKGROUND ART

It is required to further increase a recording density of magneticrecording media with the shift of a writing-reading system from ananalog system to a digital system. In particular, when video tapes andbackup tapes of computers, which face severe competition with hard discsor optical discs, cannot satisfy the above requirement, the continuanceof the products may be endangered.

To satisfy the requirement to the increase the recording density,magnetic recording media comprising a thin film of a magnetic layer areproposed. However, so-called coating type magnetic recording media,which are produced by applying a magnetic paint containing a magneticpowder dispersed in a binder on a non-magnetic support, are superior tothe thin metal film type ones in view of the productivity, and practicalreliability such as corrosion resistance. Roughly speaking, theelectromagnetic conversion characteristic of the coating type magneticrecording media has been improved by the improvement of magnetic powdersand the improvement of production methods.

In connection with the improvement of the magnetic powders, the magneticproperties are year-by-year improved in conjunction with theminiaturization of the particle size to cope with the short-wavelengthrecording. Formerly, magnetic powders such as ferromagnetic iron oxidepowder, cobalt-modified ferromagnetic iron oxide powder and chromiumoxide powder, which are used for audio tapes or domestic video tapes,are mainly used also for high density recording magnetic recordingmedia, but recently acicular metal magnetic powders having a particlesize of about 0.1 μm is proposed for the high density recording magneticrecording media.

To prevent the decrease of output due to the demagnetization in theshort wavelength recording, a coercive force has been increasedyear-by-year, and the alloy of iron-cobalt achieved a coercive force ofabout 198.9 kA/m (see U.S. Pat. No. 5,252,380, JP-A-5-234064,JP-A-6-25702, JP-A-6-139553, etc.)

In connection with the improvement of the production methods of themagnetic recording media, the use of binders having various functionalgroups, the improvement of the dispersing technique of the abovemagnetic powders, and the improvement of the calendering method afterthe application process can remarkably increase the surface smoothnessof the magnetic layers, and thus greatly contribute to the increase ofthe output in the short wavelength range (see U.S. Pat. No. 4,324,177,U.S. Pat. No. 4,952,444, JP-A-4-19815, etc.)

However, since the recording wavelength is shortened with the recentincrease of the recording density, the influences ofself-demagnetization loss in the course of writing and reading andthickness loss due to the thickness of the magnetic layer, which havenot caused any problem, increase, and thus sufficient dissolution maynot be attained. Such problems cannot be solved by the above-describedimprovement of the magnetic properties of the magnetic powders or theincrease of the surface properties achieved by the production methods ofthe media. Thus, it is proposed to decrease the thickness of themagnetic layer.

In general, the effective thickness of the magnetic layer is about onethird (⅓) of the shortest recording wavelength used in a system. Forexample, with the shortest recording wavelength of 1.0 μm, the thicknessof the magnetic layer should be about 0.3 μm. Furthermore, with theminiaturization of a cassette, the whole thickness of the magneticrecording medium should be decreased to increase a recording capacityper unit volume. Consequently, the thickness of the magnetic layershould be decreased. In addition, to increase the recording density, thearea of a writing magnetic flux, which is generated with a magnetichead, should be decreased, and thus the magnetic head is miniaturized.Therefore, the amount of the generated magnetic flux decreases.Accordingly, the magnetic layer should be made thin to cause completereversal of magnetization with the minute magnetic flux.

When the thickness of the magnetic layer is decreased, the surfaceroughness of the non-magnetic support has some influence on the surfaceof the magnetic layer and thus the surface properties of the magneticlayer tend to deteriorate. Furthermore, when the thickness of a singlemagnetic layer is decreased, it may be contemplated to decrease thesolid concentration of a magnetic paint or to decrease the amount of themagnetic paint applied. However, these methods cannot prevent defectsformed in the course of application, or achieve the increase of thefilling of the magnetic powder. Therefore, the strength of the coatedfilm may deteriorate. Accordingly, to decrease the thickness of themagnetic layer by the improvement of the production methods of themagnetic recording media, a so-called simultaneous multiple layercoating method is proposed, which comprises providing an undercoat layerbetween a non-magnetic support and a magnetic layer, and applying amagnetic paint of the upper magnetic layer while the undercoat layer isstill wet (see U.S. Pat. No. 4,863,793, U.S. Pat. No. 4,963,433, U.S.Pat. No. 5,645,917, U.S. Pat. No. 5,380,905, U.S. Pat. No. 5,496,607,etc.)

With such improvements of the coating methods, it becomes possible tothinly coat a magnetic layer having a thickness of about 1.0 μm, andsuch thin film-coating methods and the above-described improvement ofthe magnetic powders can solve the various problems such as the decreaseof the output caused by the demagnetization, which is the essentialproblem of longitudinal recording.

However, in these days, the improvements of the magnetic powders and theproduction methods of the magnetic recording media reach the limits. Inparticular, in the case of the improvement of the magnetic powders,insofar as the acicular magnetic powder is used, the practical lowerlimit of the particle size is about 0.1 μm, because when the particlesize is less than about 0.1 μm, a specific surface area of the particleincreases greatly, and thus not only the saturation magnetizationdecreases but also the dispersion of the magnetic powder in the binderbecomes very difficult.

In connection with the coercive force, signals can be recorded onmagnetic recording media having a very high coercive force because ofthe technical innovation of the magnetic heads. In particular, in thecase of the longitudinal recording system, it is desirable to increasethe coercive force to as high as possible to prevent the deteriorationof the output due to the writing and reading demagnetization, insofar asthe recorded signals can be erased with the magnetic head. Accordingly,the realistic and most effective method to increase the recordingdensity of the magnetic recording media is to increase the coerciveforce of the media.

It is effective to further decrease the thickness of the magnetic layerto suppress the influence of the decrease of the output caused by thewriting and reading demagnetization, which is the essential problem ofthe longitudinal recording. However, the thickness of the magnetic layerwill reach the limit, insofar as the above-described acicular magneticpowder having a particle size of about 0.1 μm is used. The reason is asfollows: the acicular particles are aligned in the plane direction ofthe magnetic recording medium on the average by longitudinalorientation, but some particles may be aligned in the directionperpendicular to the plane of the medium since the orientation of theparticles has distribution. When such particles are contained, theydeteriorate the surface smoothness of the medium and may increase noise.Such problems become more serious as the thickness of the magnetic layerdecreases.

When the magnetic layer is made thin, it is necessary to dilute themagnetic paint with a large amount of an organic solvent. However, theconventional miniaturized acicular magnetic powder particles tend tocause the agglomeration of the magnetic paint. In addition, since alarge amount of the organic solvent is evaporated when the appliedmagnetic paint is dried, the orientation of the magnetic powderparticles is tend to be disturbed. Thus, in the case of tape-form mediawhich are longitudinally recorded, the desired electromagneticconversion may not be attained because of the deterioration of theorientation and the surface properties, even if the magnetic layer ismade thin. Thus, it is very difficult to produce coating type magneticrecording media having the further decreased thickness of the magneticlayer, insofar as the conventional acicular magnetic powder is used,although it is known that the decrease of the thickness of the magneticlayer is effective to increase the recording characteristics of themedia in the case of longitudinal recording.

Among the already proposed magnetic powders, the barium ferrite magneticpowders having platelet particle shapes, and comprising very finemagnetic particles with a particle size of 50 nm are known (seeJP-B-60-50323, JP-B-6-18062, etc.) The shapes and particle sizes of thebarium ferrite magnetic powders are more suitable for the production ofthe thin-layer coating type magnetic recording media than the acicularmagnetic powders. However, since the barium ferrite magnetic powder isan oxide, its saturation magnetization is at most about 7.5 μWb/g, andthus it is theoretically impossible to achieve a saturationmagnetization of 12.6 μWb/g or more, which is the level of thesaturation magnetization of acicular metal or alloy magnetic powders.Therefore, when the barium ferrite magnetic powder is used, the highoutput cannot be attained since the saturation magnetization is low,although the coating type magnetic recording media comprising a thinmagnetic layer may be produced. Thus, the barium ferrite magneticpowders are not suitable for the high recording density magneticrecording media. For the above reason, the above-described acicularmagnetic powders has been dominantly used as the magnetic powders forthe high recording density magnetic recording media.

As explained above, it is a very important problem to reduce theparticle size of a magnetic powder while maintaining the coercive forceand saturation magnetization at a as high level as possible to reducethickness of the magnetic layer, which is an effective measure toincrease the recording density of the magnetic recording media. To solvesuch a problem, firstly, the magnetic characteristics of theconventional magnetic powders are discussed. In the case of thecurrently used acicular magnetic powders, the increase of the coerciveforce has a limit theoretically, since its coercive force is based onthe shape anisotropy of the acicular particles. That is, the magneticanisotropy based on the shape anisotropy is expressed by 2πI₅ wherein I₅is a saturation magnetization, and thus proportional to the saturationmagnetization. Thus, the coercive force increases as the saturationmagnetization increases in the case of the acicular magnetic powders thecoercive force of which is based on the shape anisotropy.

The saturation magnetization of a magnetic metal or alloy, for example,an Fe—Co alloy reaches the maximum near a Fe/Co ratio of 70/30, as iswell known from the Slater-Pauling's curve. Therefore, the coerciveforce also reaches the maximum at the above composition of the alloy.The acicular magnetic powder of such a Fe—Co alloy having a Fe/Co ratioof about 70/30 is already practically used.

The magnitude of the magnetic anisotropy based on the shape anisotropyis expressed by 2πI₅ as explained above. The factor is about 2π when theacicular ratio (particle length/particle diameter) of the magneticpowder is about 5 or more, but the factor quickly decreases when theacicular ratio is less than about 5. Finally the anisotropy disappears,when the particle becomes a sphere. That is, insofar as magneticmaterials of metal iron or Fe—Co alloys are used as the magneticpowders, the shape of the magnetic powder particles should be in theacicular form (needle form) from the theoretical viewpoint.

DISCLOSURE OF THE INVENTION

In view of the above circumstances, it may be inevitable to create anovel magnetic powder which is based on a new concept different from theabove-described conventional magnetic powder to attain the breakthroughof the coating type magnetic recording media. Then, based on the above,the objects of the present invention are as follows:

(I) To provide a novel magnetic powder, which is entirely different fromthe conventional magnetic powders, as a magnetic powder for a magneticrecording medium having a very thin magnetic layer.

(II) To provide a coating type magnetic recording medium comprising sucha novel magnetic powder and having excellent magnetic characteristicswhich cannot be achieved by the conventional magnetic powders.

(III) To provide a magnetic recording medium having much improvedwriting-reading characteristics in comparison with the coating typerecording media comprising the conventional magnetic powders.

To achieve the above objects, the inventors set forth the basicguideline that the properties of magnetic powders necessary toremarkably increase the recording density of the coating type magneticrecording medium having a thin magnetic layer are the followingproperties (1) through (6), and have screened raw materials and studiedmethods for the production of magnetic powders suitable for such amagnetic recording medium:

(1) A coercive force is made as high as possible in the range where therecorded signals can be erased with a magnetic head;

(2) A magnetic powder comprises iron, which has the largest saturationmagnetization among single elements and is abundantly available as anatural resource;

(3) A magnetic powder is that of a metal, a metal alloy or a metalcompound to achieve high saturation magnetization;

(4) The particle shape of a magnetic powder is close to a sphere havingthe minimum specific surface area;

(5) The particle size of a magnetic powder is made as small as possiblewhile maintaining saturation magnetization; and

(6) A magnetic powder has a uniaxial magnetic anisotropy one direction(axis) of which is a magnetization easy direction (axis).

When the present inventors have made study to develop a magnetic powderwhich satisfies all the above properties, it has been found that amagnetic powder of spherical or ellipsoidal particles having a particlesize of 5 to 200 nm and comprising metal iron, an iron alloy or an ironcompound in which a rare earth element is mainly present in the outerlayer of the magnetic powder particles satisfies all these propertiesand has good characteristics.

The term “spherical or ellipsoidal” used herein is intended to mean anyshape from a substantial sphere to a substantial ellipsoid and includesparticles having uneven surfaces or slightly deformed “spherical orellipsoidal” particles as shown in the photographs of FIGS. 1 and 2.Hereinafter, those particles are collectively referred to as “sphericalor ellipsoidal” particles.

It has also been found that, when the following method is employed as amethod for allowing the rare earth element to be present mainly in theouter surface of the magnetic powder particles, the intended magneticpowder can be produced:

Spherical or ellipsoidal particles of magnetite, cobalt-ferrite, etc.are dispersed in an aqueous solution containing at least rare earthelement ions, an aqueous solution of an alkaline material is added tothe dispersion in a molar amount sufficient for converting the rareearth element ions to a hydroxide to form a surface layer of thehydroxide on the particles of magnetite or cobalt-ferrite, and then theparticles are recovered by filtration, dried and reduced by heating.Furthermore, it has been found that an excellent high density magneticrecording medium can be obtained when a thin layer coating type magneticrecording medium is produced using the above magnetic powder of thepresent invention. In addition, it has been revealed that a magneticrecording medium comprising the above magnetic powder of the presentinvention has a high coercive force and a high magnetic flux densityalthough the magnetic powder comprises spherical or ellipsoidalultra-fine particles.

With a magnetic recording medium comprising a magnetic powder ofspherical or ellipsoidal fine particles having a very small particlesize like the magnetic powder of the present invention, magneticinteractions between the magnetic powder particles and thus it ispossible to effect very quick reversal of magnetization so that therange of the reversal of magnetization is narrowed. Accordingly, such amagnetic recording medium has much better recording characteristics thanmagnetic recording media comprising the conventional acicular magneticpowders. Furthermore, the magnetic recording medium according to thepresent invention achieves the intended effects, particularly when thethickness of the magnetic layer is 3 μm or less, and the magneticrecording medium having such a thin magnetic layer is less influenced bya demagnetizing field, and exhibits good recording properties even at acoercive force of about 80 kA/m.

As a result of the further studies based on the above findings, it hasbeen found that the magnetic powder and the magnetic recording mediumexhibit the distinguished performances, when a magnetic recording mediumhaving the following characteristics.

With the recent trend to the high recording density, the digitalrecording systems have become predominant as described above. Thus,magnetic recording media are required to have a low error rate. From theabove viewpoint, the present invention can provide a magnetic recordingmedium for digital recording having the excellent properties whichcannot be achieved by the conventional magnetic recording media, when itsatisfies the following requirements:

A) A magnetic recording medium has at least one undercoat layercomprising an inorganic powder and a binder on a non-magnetic support,and a magnetic layer comprising a magnetic powder and a binder on theundercoat layer, and the magnetic layer has an average thickness of 0.3μm or less (when the magnetization easy directions of a magnetic layerare in the longitudinal direction or randomly distributed, the averagethickness is preferably 0.2 μm or less, more preferably 0.01 to 0.15 μm,most preferably 0.01 to less than 0.1 μm);

B) The anisotropic magnetic field distribution of a magnetic layer isset in a specific range;

C) The magnetization-easy-direction of a magnetic layer is in themachine (longitudinal) direction of the medium, a coercive force is from80 to 400 kA/m, a squareness is from 0.6 to 0.9, and a saturatedmagnetic flux density is from 0.1 to 0.5 T, in the machine direction;

D) For applications in which the short wavelength characteristics areimportant, the magnetization-easy-direction is in a directionperpendicular to the magnetic layer plane, a coercive force is from 60to 320 kA/m, a squareness is from 0.5 to 0.8, and a saturated magneticflux density is from 0.1 to 0.5 T, in the perpendicular direction.

When the magnetic recording medium of the present invention is used in adisc form, it is preferable that E) the magnetization-easy-directionsare randomly distributed in the magnetic layer plane, and that acoercive force is from 45 to 320 kA/m, a squareness is from 0.4 to 0.7,and a saturated magnetic flux density is from 0.1 to 0.5 T, in anydirections in the magnetic layer plane and the direction perpendicularto the magnetic layer plane.

In the case of the magnetic recording media comprising the conventionalacicular magnetic powders, since the magnetic powder particles aremechanically oriented in a specific direction, a certain deorientationtreatment is necessary. When the magnetic powder of the presentinvention is used, such deorientation may not be necessary, which is oneof the large advantages of the present invention.

In the system using the recording with a short wavelength of 1.0 μm forthe purpose of the high density recording, the above thin layer coatingtype magnetic recording media have been improved to attain the highoutput. As a result, F) it has been found that the high output can beobtained when a P-V value (in terms of the optical interference typethree-dimensional surface roughness) is 50 nm or less.

Furthermore, the magnetic powder particles of the present invention aresubstantially not mechanically oriented so that they are aligned inparallel with the in-plane direction of the magnetic layer along themachine direction. Thus, the studies have been made to make use of theproperties of the spherical or ellipsoidal magnetic powder. As a result,it has been found that G) a high elasticity is achieved in thetransverse direction of the magnetic recording medium so that a goodhead touch, which is required to improve the properties in a helicalscan system, is achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a transmission electron microscopic photograph (magnification:300,000 times) of the samarium-iron-cobalt magnetic powder, which isproduced in Example 1.

FIG. 2 is a transmission electron microscopic photograph (magnification:300,000 times) of the yttrium-iron nitride magnetic powder, which isproduced in Example 12.

FIG. 3 is a X-ray diffraction pattern of the yttrium-iron nitridemagnetic powder, which is produced in Example 15.

FIG. 4 is a transmission electron microscopic photograph (magnification:200,000 times) of the yttrium-iron nitride magnetic powder, which isproduced in Example 15.

FIG. 5 is a transmission electron microscopic photograph (magnification:200,000 times) of the yttrium-iron nitride magnetic powder, which isproduced in Example 17.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

With the conventional acicular iron-cobalt alloy magnetic powder, whichis used for high density coating type magnetic recording medium, inconnection with the particle size (5) of the above guidelines, it isdifficult to disperse the powder in a binder, if the particle size isfurther decreased from the current particle size. In addition, the mostserious problem is that it is impossible to achieve the properties (4)and (6) at the same time, because the acicular ratio can be reduced onlyto about 5 since the coercive force is based on the shape anisotropy,that is, the acicular shape, and if the acicular ratio is furtherdecreased to less than 5, the uniaxial anisotropy deteriorates and thusthe coercive force becomes too small.

From the view point different from the magnetic powders based on theshape magnetic anisotropy, the present inventors have synthesizedvarious magnetic powders to improve the magnetic properties inaccordance with the above-described basic guideline, and checked themagnetic anisotropy of the magnetic powders. Then it has been found thatmagnetic materials comprising a transition metal such as iron or aniron-cobalt alloy containing a rare earth element have large crystallinemagnetic anisotropy, and therefore it is not necessary to form theparticles in an acicular shape and that, when the particles are in thespherical or ellipsoidal shape, the magnetic powder has a large coerciveforce in one direction. In particular, the magnetic powder exhibitsfurther improved characteristics, when the core of the magnetic powdercomprises at least one of metal iron, an iron alloy and an iron compoundwhile the rare earth element is present mainly in the outer layer of themagnetic powder particles.

Herein, the ellipsoidal magnetic powder particles mean those having aratio of the major axis to the minor axis of 2 or less, in particular,1.5 or less. Therefore, the magnetic powder particles of the presentinvention have essentially different shapes from those of theconventional powder particles for the magnetic recording media.

Among the magnetic materials comprising a rare earth element and atransition metal, rare earth element-iron-boron magnetic materials areknown as high performance magnetic materials comprising particles of asubmicron order, which are produced by powder metallurgical methods.

For example, a neodymium-iron-boron magnetic material for a permanentmagnet has a composition represented by Nd₂Fe₁₄B, and a very largecoercive force of 800 kA/m or more. Such a coercive force is too highfor magnetic recording media, but the present inventors found that thecoercive force of such a magnetic material can be adjusted to a rangesuitable for magnetic recording media by controlling the contents of arare earth element and boron (see JP-A 2001-181754).

The rare earth element-iron-boron magnetic material having thecomposition of Nd₂Fe₁₄B has the very high coercive force as describedabove. In the previous invention described above, it was also found thatwhen samarium (Sm) or yttrium (Y) is used as a rare earth element inplace of Nd, the magnetic powder has a coercive force sufficient for usein the magnetic recording media (see JP-A 2001-181754).

In the field of permanent magnets comprising neodymium-iron-boroncompounds, the contents of the rare earth element, the transition metaland boron are optimized to achieve as high coercive force as possible,but the present inventors have found that a coercive force which islower than that of the permanent magnets but is suitable for magneticrecording media can be attained by controlling the contents of the rareearth element and boron. Furthermore, as a result of the detailed studyof the mechanism for achieving such a coercive force, it has been foundthat, when the rare earth element is present mainly in the outer layerof the magnetic powder particles, the magnetic powder has a coerciveforce suitable for magnetic recording in the absence of boron. Thus, thepresent invention has been completed.

That is, the present inventors firstly notice and successfully use therare earth element-iron-boron magnetic materials, which attractedattentions as permanent magnet materials, as magnetic powders formagnetic recording media in a lower coercive force range than that ofthe permanent magnet materials, and develop the completely new field ofmagnetic materials.

As described above, the present inventors fully analyzed the rare earthelement-iron-boron magnetic materials, which are completed technicallyand theoretically, and intended to develop a magnetic powder formagnetic recording media based on this magnetic materials. As a result,it has been found that a high coercive force is achieved in the rangewhere the recorded signals can be erased with a magnetic head, and alsothe excellent electromagnetic conversion characteristics as the thinlayer coating type magnetic recording media can be attained, when amagnetic powder comprises a rare earth element and iron but no boron,contains a rare earth element mainly in the outer layer of the magneticpowder particles and has a spherical or ellipsoidal shape with aparticle size of 5 to 200 nm. As the rare earth element contained insuch magnetic powders, at least one element selected from the groupconsisting of yttrium, ytterbium, cesium, praseodymium, samarium,lanthanum, europium, neodymium and terbium is used. Among them, whenneodymium, samarium, yttrium and/or terbium are used, a high coerciveforce can be easily attained. In addition, the above effects aremaximized when an iron-cobalt alloy is used as a transition metal andthe contents of iron and cobalt are in an atomic ratio range of 3:97 to40:60 (a ratio of cobalt to iron).

As the transition metal, metal iron or an alloy comprising iron ispreferable, although an iron-containing compound such as iron nitridecan be used apart from such an alloy.

The magnetic powder of the present invention exhibits the excellentmagnetic properties for high density magnetic recording media, when ithas a particle size of 5 to 200 nm. With the conventional acicularmagnetic powder, the lower limit of the average particle size is about0.1 μm to maintain the high coercive force. However, the magnetic powderof the present invention can be made very fine to have the averageparticle size of at least 5 nm, and such fine particles can exhibit goodmagnetic properties, since the coercive force is mainly based on thecrystalline magnetic anisotropy. In particular, the average particlesize is preferably at least 8 nm, more preferably at least 10 nm.

When the average particle size of the magnetic powder is too large, thefilling properties of the magnetic powder in the magnetic layerdeteriorate, and also the surface properties deteriorate when themagnetic layer is made thin. In addition, the particle noise due to theparticle size increases when the magnetic recording medium is producedusing such a magnetic powder having a large average particle size.Accordingly, the average particle size should be 200 nm or less and ispreferably 100 nm or less, more preferably 50 nm or less. When theaverage particle size is adjusted in such a range, the very high fillingproperties are attained, and the excellent saturated magnetic fluxdensity is achieved.

Herein, the average particle size of the magnetic powder is obtained bymeasuring the particle sizes of 500 particles in the transmissionelectron microscopic (TEM) photograph taken at a magnification of100,000 times and averaging the measured particle sizes, or by measuringthe particle sizes of 300 particles in the transmission electronmicroscopic photograph taken at a magnification of 200,000 times andaveraging the measured particle sizes. The former method is used unlessotherwise indicated.

When the iron alloy is used among the metal iron, iron alloy and ironcompound which contribute to the increase of the saturationmagnetization in the magnetic powder comprising the rare earth elementand iron, examples of metals which form alloys with iron includemagnetic transition metals such as Mn, Zn, Ni, Cu, Co, etc. Among them,Co and Ni are preferable. In particular, Co is preferable. As describedabove, when the iron-cobalt alloy is used, a large saturationmagnetization is obtained as can be seen from the Slater-Pauling'scurve. In addition, the alloying of cobalt contributes to the increaseof a coercive force.

As the iron compound, iron nitride, which comprises nitrogen, or ironnitride a part of iron atoms are replaced with a transition metalelement is preferable. A various types of iron nitride having differentcompositions are known. Particularly, iron nitride of the formula:Fe₁₆N₂ or Fe₁₆N₂ a part of iron atoms of which are replaced with atransition metal element is preferable to achieve the high coerciveforce and the high saturation magnetization.

The amount of the rare earth element contained in the magnetic powder ofthe present invention is from 0.2 to 20 atomic %, preferably from 0.5 to15 atomic %, more preferably from 1.0 to 10 atomic %, based on thetransition metals. In the rare earth element-iron-boron magnetic powder,which is the subject of the previous invention by the present inventors,it is preferable that the rare earth element, iron and boron areuniformly contained in the magnetic powder to achieve the high coerciveforce. The magnetic powder of the present invention is greatly differentfrom the magnetic powder of the previous invention in that the rareearth element is present mainly in the outer layer of the magneticpowder particles to achieve the high coercive force. As a result, acoercive force suitable for magnetic recording media can be attained inthe absence of boron which has been considered to be essential toachieve the high coercive force. Since the magnetic powder contains therare earth element in the outer layer of the particles in a highconcentration and exhibits the coercive force by the surface effectthereof, the small content of the rare earth element, such as 10 atomic% or less, based on the amount of the transition metal can achieve acoercive force of 80 to 400 kA/m which is suitable as a magnetic powderfor high performance magnetic recording media.

To produce such a magnetic powder, the novel method of the presentinvention disperses spherical or ellipsoidal particles of magnetite orcobalt ferrite in an aqueous solution containing rare earth element ionsto form a dispersion, an alkaline aqueous solution is added to thedispersion to form the hydroxide of the rare earth element over thesurfaces of the magnetite or cobalt ferrite particles, and then theparticles are recovered by filtration, dried and reduced by heating.

Now, the particle shape of the above magnetic powder is explained fromthe viewpoint of the dispersion of the powder in the magnetic paint andthe properties required to form the thin magnetic layer.

In the case of the conventional acicular magnetic powders, the particlesize is decreased to improve the recording properties such as thedecrease of noise. As a result, the specific surface area of theparticles inevitably increases. Thus, the interaction with the binderincreases so that it becomes difficult to obtain a homogeneousdispersion when the magnetic powder is dispersed in the binder.Furthermore, when the magnetic paint dispersion is diluted with a largeamount of an organic solvent to apply a thin layer, the magnetic powderparticles tend to agglomerate, and therefore the orientation and surfaceproperties deteriorate. Consequently, the particle size of the magneticpowder, which can be used in the production of the coating type magneticrecording media, is limited.

In contrast to the conventional magnetic powders, the magnetic powder ofthe present invention has the spherical or ellipsoidal shape, and thusit can have a shape close to a sphere having the smallest specificsurface area. Therefore, in comparison with the conventional magneticpowders, the magnetic powder of the present invention has a smallinteraction with the binder and can provide a magnetic paint with goodflowability. If the magnetic powder particles are agglomerated, theredispersion of the particles is easy. Thus, the magnetic powder of thepresent invention can provide the magnetic paint which is particularlysuitable for the formation of the thin magnetic layer. As a result, themagnetic powder having the average particle size of about 5 nm can bepractically used.

The decrease of the thickness of the magnetic layer is effective tosuppress the decrease of the output due to the writing and readingdemagnetization, which is the essential problem of the longitudinalrecording. Insofar as the acicular magnetic powder having the particlesize of about 0.1 μm is used, the thickness of the magnetic layer islimited, because the acicular particles are aligned in the planedirection of the magnetic recording medium on the average by theorientation in the magnetic field, but some particles may be aligned inthe direction perpendicular to the plane of the medium since theorientation of the particles has distribution. When such particles arecontained, they protrude from the surface of the magnetic layer anddeteriorate the surface properties of the medium and may increase noise.Such problems become more serious as the thickness of the magnetic layerdecreases. Thus, it is difficult to produce the coated film having athickness of about 0.3 μm or less and also the smooth surface, insofaras the acicular magnetic powder is used.

When an undercoat layer is provided between the non-magnetic support andthe magnetic layer to reduce the thickness of the magnetic layer asexplained below, and the undercoat layer is formed by the simultaneousmultiple layer coating method in which the magnetic paint for themagnetic layer containing the dispersed acicular magnetic powder iscoated over the undercoat layer while the undercoat layer is still wet,the magnetic powder is entrained by the undercoat layer so that theacicular magnetic powder particles tend to penetrate into the undercoatlayer at the interface between the magnetic powder and the undercoatlayer. Thus, the orientation of the magnetic powder particles is furtherdisturbed, so that the desired squareness is not attained, and thesurface smoothness of the magnetic layer deteriorates. Accordingly, theabove problem may be one of the causes for a bar to the increase of therecording density by the thin layer coating when the acicular magneticpowder is used.

In contrast to the acicular magnetic powder, the magnetic powder of thepresent invention has a small particle size and also the spherical orellipsoidal particle shape and can have the particle shape close to thesphere. Therefore, the powder particles do not protrude from the surfaceof the magnetic layer. When the undercoat layer is provided, thepenetration of the magnetic powder particles into the undercoat layercan be suppressed in contrast with the acicular magnetic powder.Accordingly, the magnetic layer having the extremely smooth surface canbe formed.

As the thickness of the magnetic layer decreases, the magnetic flux fromthe magnetic layer decreases and thus the output decreases. Since themagnetic powder of the present invention has the spherical orellipsoidal particle shape and can have the particle shape close to thesphere, it has an advantage such that the magnetic powder can becontained in the magnetic layer at a higher filling rate than theacicular magnetic powder and thus the high magnetic flux density can beeasily attained.

Furthermore, with respect to the saturation magnetization, in general,the metal or metal alloy magnetic powders have the larger specificsurface area as the particle size decreases, so that the ratio of thesurface oxide layer which does not contribute to the saturationmagnetization increases, while the magnet part contributing to thesaturation magnetization decreases. That is, as the particle sizedecreases, the saturation magnetization decreases. This tendency isremarkable with the acicular magnetic powders, and the saturationmagnetization suddenly decreases, when the major axis of the acicularparticle is 0.1 μm or less. Such decrease of the saturationmagnetization is taken into consideration, when the limit of the usableparticle size is determined. Since the magnetic powder of the presentinvention has the particular or ellipsoidal particle shape, the specificsurface area is minimum among the particles having the same volume.Therefore, the magnetic powder of the present invention can maintain thehigh saturation magnetization in spite of the fine particle.

In the present invention, the particle shape of the rare earthelement-iron magnetic powder is expressed by “spherical or ellipsoidal”.This intends to include any shape from substantially particulate to theellipsoid including any intermediate shapes between the particle and theellipsoid. That is, the above expression is intended to exclude the“acicular” shape of the conventional magnetic powders. Among variousshapes, a sphere having the smallest specific surface area to anellipsoid are preferable. The particle shapes can be observed using anelectron microscope like in the measurement of the particle size.

As explained above, the magnetic powder of the present invention has thesaturation magnetization, coercive force, particle size and particleshape, all of which are essentially suitable to form the thin magneticlayer, and particularly good writing-reading characteristics can beachieved, when the magnetic recording medium having the magnetic layerwith an average thickness of 0.3 μm or less is produced using such amagnetic powder. Among the magnetic powders of the present invention,those having a saturation magnetization of 10 to 25 μWb/g are preferablyused to improve the characteristics in the high recording density rangein the case of the magnetic recording medium having the magnetic layerwith the average thickness of 0.3 μm or less.

Herein, the coercive force and saturation magnetization of the magneticpowder are values, which are measured with a sample-vibration typemagnetometer at 25° C. in an applied magnetic field of 1,273.3 kA/m andcompensated using a standard sample.

The above magnetic powder of the present invention may be prepared bythe following method:

The spherical or ellipsoidal particles of magnetite or cobalt ferrite,which are beforehand prepared, are dispersed in water. To thisdispersion, the rare earth ion of, for example, neodymium, samarium,yttrium, etc. is dissolved. The rare earth element ion may be added inthe form of a salt of the rare earth element such as a nitrate, etc.

Then, an aqueous solution of an alkaline material is added to thedispersion in a molar amount sufficient for converting the rare earthelement ion to a hydroxide. Thereby, the hydroxide of the rare earthelement is deposited on the surface of the particles of magnetite orcobalt ferrite. In this step, it is important to adjust the amount ofthe alkaline material at a molar amount sufficient for converting therare earth element ion to a hydroxide. When the amount of the alkalinematerial is too small, the rare earth element ion may not be easilydeposited on the surface of the magnetite or cobalt ferrite particles inthe form of a hydroxide. When the amount of the alkaline material is toolarge, the hydroxide of the rare earth element tends to grow so that thesurface of the magnetite or cobalt ferrite particles may not beuniformly covered. The hydroxide of the rare earth element is a keymaterial for achieving the high coercive force and also function as asintering-preventing agent. Therefore, the uniform formation of thehydroxide on the surface of the magnetite or cobalt ferrite particles isvery important in the present invention.

Examples of the alkaline material include the hydroxide of alkali metalsand alkaline earth metals.

The magnetite or cobalt ferrite particles carrying the hydroxide of therare earth element are washed with water, recovered by filtration, driedand then reduced by heating to obtain spherical or ellipsoidal magneticpowder particles a having a particle size of 5 to 200 nm, a coerciveforce of 80 to 400 kA/m and a saturation magnetization of 10 to 25μWb/g. The final particle size and shape of the magnetic powder obtainedare substantially dependent on those of the magnetite or cobalt ferriteparticles used as the raw material.

In the method for producing a magnetic powder according to the presentinvention, what is greatly different from other production method of amagnetic powder is that three properties, namely, the particle shape,the saturation magnetization and the coercive force, which are the mostimportant properties of the magnetic powder, are independentlycontrolled in the production process. That is, the particle size andshape are controlled with those of the magnetite or cobalt ferriteparticles used as the raw material, while the saturation magnetizationis controlled with the composition of the metal or alloy or thecompound, which is obtained after reduction and the degree of oxidizingstabilization treatment after reduction, and the coercive force iscontrolled with the amount of the rare earth element contained in themagnetic powder.

When iron nitride is used as an iron component, it can be formed bynitriding iron with ammonia gas after reduction by heating. As theammonia gas, pure ammonia gas may be used, while ammonia gas is mixedwith a carrier gas such as nitrogen gas, hydrogen gas, etc.

The primary constituent elements of magnetite or cobalt ferrite areiron, or iron and cobalt. Besides these transition metals, othertransition metal ions such as Mn, Zn, Ni, Cu, etc. may be contained inthe magnetic powder. These other transition metals are preferablycontained in the magnetite or cobalt ferrite particles.

According to the present invention, the core of the magnetic powderparticles comprises the metal iron or the iron alloy with the abovetransition metal(s), or the iron compound, while the rare earth elementis present mainly in the outer layer of the magnetic powder particles.Therefore, the content of the rare earth element is from 0.2 to 20atomic %, preferably from 0.5 to 15 atomic %, more preferably from 1.0to 10 atomic %, based on the-transition metal. According to the presentinvention, the intended high coercive force can be attained with such asmall amount of the rare earth element.

Hereinafter, a preferred embodiment of a substantially spherical orellipsoidal rare earth element-iron magnetic powder containing the rareearth element mainly in the outer layer of magnetic powder particles, inwhich the core comprises the iron compound selected from Fe₁₆N₂ andFe₁₆N₂ a part of iron atoms of which are replaced with-other transitionmetal, will be explained

The rare earth element-iron nitride magnetic powder of the presentinvention comprises substantially spherical or ellipsoidal magneticpowder particles in which the rare earth element is present mainly inthe outer layer of the magnetic powder particles. Preferably, such amagnetic powder has an average particle size of 5 to 50 nm, particularly10 to 50 nm, and an average acicular ratio (an averaged ratio of alonger axis length (diameter) to a shorter axis length (diameter)) of 2or less, particularly 1.5 or less. The content of the rare earth elementis preferably from 0.05 to 20 atomic % based on the ion atoms in themagnetic powder. A BET specific surface area of the particles ispreferably from 40 to 100 m²/g.

The above rare earth element-iron nitride magnetic powder can beproduced by supplying an oxide or hydroxide of iron as a raw material,coating the particles of the oxide or hydroxide of iron with the rareearth element, reducing them by heating, and then nitriding iron at atemperature lower than the reducing temperature.

In the rare earth element-iron nitride magnetic powder of the presentinvention, a content of the rare earth element is preferably from 0.05to 20 atomic %, more preferably from 0.2 to 20 atomic %, particularlypreferably from 0.5 to 15 atomic %, most preferably from 1.0 to 0.10atomic %, based on the amount of iron. The content of nitrogen ispreferably from 1.0 to 20 atomic %, more preferably from 1.0 to 12.5atomic %, particularly preferably from 3 to 12.5 atomic %, based on theamount of iron.

When the content of the rare earth element is too small, thecontribution of the rare earth element to the magnetic anisotropydecreases, and large magnetic powder particles tend to form because ofsintering in the reducing process so that a particle size distributionmay deteriorate. When the content of the rare earth element is toolarge, the amount of unreacted rare earth element, which does notcontribute to the magnetic anisotropy, increases so that the magneticproperties, in particular, the saturation magnetization tend toexcessively deteriorate.

When the content of nitrogen is too small, the amount of the Fe₁₆N₂phase decreases so that the coercive force is not increased. When thecontent of the nitrogen is too large, non-magnetic nitride tends to beformed so that the coercive force is not increased and further thesaturation magnetization tends to excessively decrease.

The shape of the rare earth element-iron nitride magnetic powder of thepresent invention is substantial sphere or ellipsoid having an acicularratio of 2 or less, in particular, a substantial sphere having anacicular ratio of 1.5 or less. The rare earth element-iron nitridemagnetic powder of the present invention has an average particle size of5 to 50 nm, in particular, 10 to 50 nm. When the particle size is toosmall, the dispersibility of the magnetic powder tends to deteriorate inthe preparation of a magnetic paint. Furthermore, the magnetic powdermay become thermally unstable, and the coercive force may change overtime. When the particle size is too large, it may increase the noise andalso the magnetic layer may not have a smooth surface.

The particle size of the rare earth element-iron nitride magnetic powderis determined by measuring the particle sizes of 300 particles in thetransmission electron microscopic photograph taken at a magnification of200,000 times and averaging the measured particle sizes.

The rare earth element-iron nitride magnetic powder of the presentinvention preferably has a saturation magnetization of 10 to 20 μWb/g,more preferably 11.3 to 19.5 μWb/g, particularly preferably 12.6 to 18.2μWb/g, and a coercive force of 119.4 to 318.5 kA/m, more preferably159.2 to 278.6 kA/m.

The rare earth element-iron magnetic powder of the present inventionpreferably has a BET specific surface area of 40 to 100 m²/g. When theBET specific surface area is too small, the particle size becomes toolarge so that the magnetic recording medium comprising such a magneticpowder tend to have a high particle noise and the surface smoothness ofthe magnetic layer decreases so that the reproducing output tends todecrease. When the BET specific surface area is too large, it isdifficult to prepare a uniformly dispersed magnetic paint due to theagglomeration of the magnetic powder particles. When such a magneticpowder is used to produce a magnetic recording medium, the orientationmay decrease or the surface smoothness may deteriorate.

As described above, the rare earth element-iron nitride magnetic powderof the present invention has the excellent properties as the magneticpowder for magnetic recording media. In addition, this magnetic powderhas good storage stability. Thus, when this magnetic powder or magneticrecording media comprising this magnetic powder is stored underhigh-temperature high-humidity conditions, it does not suffer from thedeterioration of the magnetic properties. Therefore, this magneticpowder is suitable for use in magnetic recording media for high-densityrecording.

In the case of the rare earth element-iron nitride magnetic powder, thepresence of the rare earth element inside the magnetic powder particlesis not excluded. In such a case, the magnetic powder particles have amulti-layer structure having an inner layer and an outer layer, and therare earth element is dominantly present in the outer layer near thesurface of the particle. Preferably, at least 50%, more preferably atleast 70% of the rare earth element may be contained in the outer layerof the particle. When the magnetic powder has such a structure, the ironphase of the inner layer usually comprises the Fe₁₆N₂ phase. However, itis not necessary for the inner layer to consist of the Fe₁₆N₂ phase, butthe inner layer may comprise a mixture of the Fe₁₆N₂ phase and an α-Fephase. The latter is sometimes advantageous since a desired coerciveforce can be easily achieved by adjusting the ratio of the Fe₁₆N₂ phaseto the α-Fe phase.

As described above, the rare earth element may be yttrium, ytterbium,cesium, praseodymium, samarium, lanthanum, europium, neodymium, terbium,etc. Among them, yttrium, samarium or neodymium can greatly increase thecoercive force and effectively serves to the maintenance of the particleshape in the reducing step. Thus at least one of yttrium, samarium andneodymium is preferably used.

Together with such a rare earth element, other element suchasphosphorus, silicon, aluminum, carbon, calcium, magnesium, etc. may becontained in the magnetic powder. When at least one of silicon andaluminum, which effectively prevent sintering, is used in combinationwith the rare earth element, a high coercive force is attained.

As described above, the rare earth element-iron nitride magnetic powdermay be produced using an oxide or hydroxide of iron (e.g. hematite,magnetite, goethite, etc.) as a raw material. The average particle sizeof the raw material is selected by taking into consideration the volumechange of the particle in the reducing and nitriding steps, and usuallyfrom about 5 to 100 nm.

The rare earth element is adhered or deposited on the surface of the rawmaterial particles. Usually, the raw material is dispersed in an aqueoussolution of an alkali or an acid. Then, the salt of the rare earthelement is dissolved in the solution and the hydroixde or hydrate of therare earth element is precipitated and deposited on the raw materialparticles by a neutralization reaction, etc.

The amount of the rare earth element is usually from 0.05 to 20 atomic%, preferably from 0.2 to 20 atomic %, more preferably from 0.5 to 15atomic %, particularly preferably from 1.0 to 10 atomic %, based on theiron atoms in the magnetic powder.

In addition to the rare earth element, a compound of silicon or aluminumwhich prevents the sintering of the particles is dissolved in a solventand the raw material is dipped in the solution so that such an elementcan be deposited on the raw material particles together with the rareearth element. To effectively carry out the deposition of such anelement, an additive such as a reducing agent, a pH-buffer, a particlesize-controlling agent, etc. may be mixed in the solution. Silicon oraluminum may be deposited at the same time as or after the deposition ofthe rare earth element.

Then, the raw material particles on which the rare earth element andoptionally other element are deposited are reduced by heating them inthe atmosphere of a reducing gas. The kind of the reducing gas is notlimited. Usually a hydrogen gas is used, but other reducing gas such ascarbon monoxide may be used.

A reducing temperature is preferably from 300 to 600° C. When thereducing temperature is less than 300° C., the reducing reaction may notsufficiently proceed. When the reducing temperature exceeds 600° C., theparticles tend to be sintered.

After the reduction of the particles, they are subjected to thenitriding treatment. Thereby, the rare earth element-iron nitridemagnetic powder of the present invention is obtained. The nitridingtreatment is preferably carried out with a gas containing ammonia. Apartfrom pure ammonia gas, a mixture of ammonia and a carrier gas (e.g.hydrogen gas, helium gas, nitrogen gas, argon gas, etc.) may be used.The nitrogen gas is preferable since it is inexpensive.

The nitriding temperature is preferably from about 100 to 300° C. Whenthe nitriding temperature is too low, the particles are not sufficientlynitrided so that the coercive force may insufficiently be increased.When the nitriding temperature is too high, the particles areexcessively nitrided so that the proportion of Fe₄N and Fe₃N phasesincreases and thus the coercive force may rather be decreased and thesaturation magnetization tends to excessively decrease.

The nitriding conditions are selected so that the content of thenitrogen atoms is usually from 1.0 to 20 atomic %, preferably from 1.0to 12.5 atomic %, more preferably from 3 to 12.5 atomic %, based on theamount of iron in the rare earth element-iron nitride magnetic powderobtained.

In the magnetic recording medium of the present invention, the magneticlayer is formed by mixing and dispersing the magnetic powder of thepresent invention, the binder and usually additives such as an abrasive,a dispersant, a lubricant, etc. as well as carbon black in an organicsolvent to obtain the magnetic paint, applying the magnetic paint on thenon-magnetic support with or without inserting the undercoat layerbetween them, and drying the applied magnetic paint.

The binder used in the magnetic layer is not limited and may be acombination of a polyurethane resin and at least one resin selected fromthe group consisting of vinyl chloride resins, vinyl chloride-vinylacetate copolymer resins, vinyl chloride-vinyl alcohol copolymer resins,vinyl chloride-vinyl acetate-maleic anhydride copolymer resin, vinylchloride-hydroxyalkyl acrylate copolymer resins and nitrocelluloseresins. Among them, the polyurethane resin and the vinylchloride-hydroxyalkyl acrylate copolymer resin are preferably used incombination. Examples of the polyurethane resin include polyesterpolyurethane, polyether , polyetherpolyester polyurethane, polycarbonatepolyurethane, polyesterpolycarbonate polyurethane, etc.

Preferably, the binder resins have a functional group to improve thedispersibility of the magnetic powder and increase the filling rate ofthe magnetic powder. Examples of the functional group include —COOM,—SO₃M, —OSO₃M, —P═O(OM)₃, —O—P—O(OM)₂ (wherein M is a hydrogen atom, analkali metal or an amine group), —OH, —NR₂, —N′R₃ (wherein R is ahydrogen atom or a hydrocarbon group), an epoxy group, etc. When two ormore resins are used in combination, they preferably have the samefunctional group.

The amount of the binder is usually from 5 to 50 wt. parts, preferablyfrom 10 to 35 wt. parts, based on 100 wt. parts of the magnetic powder.In particular, when the vinyl chloride resin is used as the binder, itsamount is from 5 to 30 wt. parts, and when the polyurethane resin isused, its amount is from 2 to 20 wt. parts. Most preferably, the vinylchloride resin and the polyurethane resin are used in combination in theabove amounts.

It is preferable to use the binder in combination with a thermallycuring crosslinking agent which bonds with the functional group in thebinder to crosslink the binder resin. Preferable examples of thecrosslinking agent include polyisocyanates such as isocyanates (e.g.tolylenediisocyanate, hexamethylenediisocyanate, isophoronediisocyanate,etc.), reaction products of such isocyanates with a compound having aplurality of hydroxyl groups (e.g. trimethyloipropane, etc.),condensation products of such isocyanates, and the like. The amount ofthe crosslinking agent is usually from 15 to 70 wt. parts per 100 wt.parts of the binder.

To increase the strength of the magnetic layer, abrasives with highhardness is preferably used. As the abrasive, there may be usedmaterials having a Mohs hardness of at least 6, for example, α-aluminahaving an aiphatization degree of at least 90%, β-alumina, siliconcarbide, chromium oxide, cerium oxide, α-iron oxide, corundum,artificial diamond, silicon nitride, silicon carbide, titanium carbide,titanium oxide, silicon dioxide, boron nitride, and mixtures thereof.Furthermore, complexes of these abrasives (for example, an abrasive theparticle surfaces of which are treated with other abrasive) may be used.Among them, alumina particles are preferred, and examples of thecommercially available alumina particles are “AKP-10”, “AKP-12”,“AKP-15”, “AKP-30”, “AKP-50”, “HIT-82” and “HIT-60 (all available fromSumitomo Chemical Co., Ltd.), “UB 40B” (manufactured by MurakamiIndustries, Ltd.), and the like.

The particle size of the abrasive is preferably from 0.01 to 1 μm. Ifnecessary, abrasives having different particles sizes, or a singleabrasive having a particle size distribution may be used to achieve thesame effects. The particle shape of the abrasive may be a needle form, asphere, a cube, etc. and those having a corner in the shape arepreferable since the abrasive having such shape has high abradingproperties. The amount of the abrasive is usually from 6 to 20 wt.parts, preferably from 8 to 15 wt. parts, per 100 wt. parts of themagnetic powder from the viewpoint of the electromagnetic conversionproperties and the contamination of the magnetic head.

Examples of the method for adding the abrasives include a methodcomprising adding the abrasive directly to the magnetic paint containingthe magnetic powder and the binder in the kneading step using a kneaderor the pre-mixing step in the course of the preparation of the magneticpaint; a method comprising separately preparing a dispersion containingthe abrasive and adding the dispersion to the magnetic paint; etc. Theformer method, which requires no separate step, is preferably used fromthe viewpoint of the productivity.

As one of the additives, a surfactant is preferably used as adispersant. Examples of the surfactant include nonionic surfactants suchas alkylene oxide base surfactants, glycerin base surfactants, glycidolbase surfactants, alkylphenol-ethylene oxide adducts, etc.; cationicsurfactants such as cyclic amines, ester amides, quaternary ammoniumsalts, hydantoin derivatives, heterocyclic compounds, phosphonium salts,sulfonium salts, etc.; anionic surfactants having an acid group such asa carboxylic acid group, a sulfonic acid group, a phosphoric acid group,a sulfate ester group, aphosphate ester group, etc.; amphotericsurfactants such as amino acids, aminosulfonic acid, sulfate orphosphate esters of aminoalcohols, etc.; and the like.

A further additive contained in the magnetic layer is preferably alubricant. Examples of the lubricant include known fatty acids, fattyacid esters, fatty acid amides, metal salts of fatty acids, hydrocarbon,and mixtures of two or more of them. Among them, fatty acids having atleast 10 carbon atoms, preferably 12 to 24 carbon atoms are preferablyused. Such fatty acids partly adhere to the magnetic powder tofacilitate the dispersing of the magnetic powder and also soften thecontact between the medium and the magnetic head in the initial abradingstate to decrease a coefficient of friction. Thus, the fatty acidscontribute to the suppression of the head contamination.

The fatty acids may be linear or branched and unsaturated or saturatedones. The linear fatty acids are preferable since they have goodlubrication properties. Examples of the linear fatty acids includelauric acid, myristic acid, stearic acid, palmitic acid, oleic acid,isostearic acid, etc.

The amount of the dispersant is preferably from 0.5 to 5 wt. parts, morepreferably from 1 to 4 wt. parts, per 100 wt. parts of the magneticpowder. The amount of the lubricant is preferably from 0.2 to 10 wt.parts, more preferably from 0.5 to 5 wt. parts, per 100 wt. parts of themagnetic powder.

To decrease the coefficient of friction of the magnetic layer andprevent the electrostatic charge, carbon black is preferably used.Examples of the carbon black include furnace black for rubbers, thermalblack for rubbers, carbon black for coloring, acetylene black, etc. Thecarbon black preferably has a specific surface area of 5 to 500 m²/g, aDBP oil absorption of 10 to 400 ml/100 g, a particle size of 5 to 300nm, pH of 2 to 10, a water content of 0.1 to 10 wt. %, and a tap densityof 0.1 to 1 g/cc. Examples of the commercially available carbon blackare “SEVACARB MTCI” (manufactured by Columbian Carbon), “Thermax PowderN-991” (manufactured by CANCARB), etc.

The amount of the carbon black added is usually 3 wt. % or less based onthe magnetic powder.

In the formation of the magnetic layer, any conventionally used organicsolvent may be used as the organic solvent which is used in thepreparation of the magnetic paint and the lubricant solution. Examplesof the organic solvent include aromatic solvents (e.g. benzene, toluene,xylene, etc.), ketone solvents (e.g. acetone, cyclohexanone, methylethyl ketone, methyl isobutyl ketone, etc.), acetate solvents (e.g.ethyl acetate, butyl acetate, etc.), carbonate solvents (e.g. dimethylcarbonate, diethyl carbonate, etc.), alcohols (e.g. ethanol,isopropanol, etc.), hexane, tetrahydrofuran, dimethylformamide, and soon.

In the production of the magnetic recording media of the presentinvention, any known method for the preparation of paints can be used toform the magnetic layer and the undercoat layer which will be describedbelow. In particular, a kneading process using a kneader or the like anda primary dispersing process are preferably used in combination. In theprimary dispersion process, a sand mill is preferably used since thedispersibility of the magnetic powder is improved and also the surfaceproperties of the magnetic layer can be controlled.

In the primary dispersing process, zirconia beads having high hardnessare preferably used as dispersing media. Examples of the zirconia beadsare TORAYCERAM (manufactured by TORAY), ZIRCONIA BALL (manufactured byNIPPON KAGAKU TOGYO), etc. The dispersing time may be suitably adjustedin the range between 30 and 100 minutes in terms of the residence timeof the paint.

The magnetic properties of the magnetic layer, which is formed asdescribed above and contains the magnetic powder, the binder and theother components, preferably include a coercive force of from 80 to 400kA/m, particularly from 95 to 320 kA/m, and a saturated magnetic fluxdensity of from 0.1 to 0.5 T, particularly from 0.2 to 0.4 T.

Herein, the above magnetic properties are measured using asample-vibration type magnetometer at 25° C. in an external magneticfield of 1273.3 kA/m like in the case of the magnetic powder with asample prepared by laminating 20 pieces of magnetic recording media andblanking the laminate to a disc of 8 mm in diameter. The measured valuesare compensated using the standard sample.

As explained in the above, when the magnetic powder of the presentinvention is used in the production of the magnetic recording mediaaccording to the present invention, it does not require such a largesaturation magnetization as required by the acicular magnetic powder.When the signals are recorded on the magnetic recording media, thedomains of the reversal of magnetization in the media do not contributeto the output. Thus, such domains are preferably made as small aspossible. However, with the conventional acicular magnetic powder thecoercive force of which is based on the shape magnetic anisotropy, themagnetic interaction among the magnetic powder particles increases asthe saturation magnetization increases, and thus a large static magneticenergy is accumulated when the reversal of magnetization is effectedquickly. Therefore, the reversal of magnetization should be effectedslowly. As a result, the domains of the reversal of magnetizationextend. In contrast, the coercive force of the magnetic powder is basedon the crystalline magnetic anisotropy, and thus the magneticinteraction among the magnetic powder particles is low. As a result, thereversal of magnetization can be effected quickly. Thus, the domains ofthe reversal of magnetization are narrowed and the large output can beobtained even with the relatively small saturation magnetization.

According to the magnetic recording media of the present invention,their properties are remarkably exhibited to solve the decrease of theoutput due to the demagnetization, which is the essential problem of thelongitudinal recording, when the magnetic layer is made thin to have theaverage thickness of 0.3 μm or less. The thickness of the magnetic layeris determined depending on the recording wavelength used. The effects ofthe present invention can be particularly exhibited when the presentinvention is applied to the recording system using the shortestrecording wavelength of 1.0 μm or less. For example, with the systemusing the shortest recording wavelength of 0.6 μm such as DLT-4, theaverage thickness of the magnetic layer is preferably about 0.2 μm, andwith the system using the shortest recording wavelength of 0.33 μm suchas DDS-3, the average thickness of the magnetic layer is preferablyabout 0.1 μm. Thus, the present invention is preferably applied to thesystems requiring the very thin magnetic layers. Furthermore, it isexpected that the magnetic layer would be made very thin, for example,less than 0.1 μm, if the recording wavelength decreases in future. Thistendency is remarkable when the magnetization-easy-directions are in thelongitudinal direction or randomly distributed. From the viewpoint ofpractical production, the lower limit of the thickness of the magneticlayer may be preferably 0.01 μm.

The anisotropic magnetic field distribution of the magnetic recordingmedia of the present invention is preferably 0.6 or less in the case ofthe longitudinally oriented magnetic recording media. When theanisotropic magnetic filed distribution of the magnetic recording mediawith the longitudinal orientation is 0.6 or less, the dispersibility andorientation properties of the fine particles of the magnetic powderaccording to the present invention are improved, so that the output atthe short wavelength is increased and the error rate is improved evenwhen the coercive force is the same.

In general, the value of the anisotropic magnetic filed distributiondecreases as the orientation properties of the magnetic powderincreases, since the former depends on the latter. However, the magneticpowder of the present invention exhibits the good anisotropic magneticfiled distribution even at random distribution, since it has the betterparticle size distribution than the conventional acicular magneticpowder.

When the magnetic recording media of the present invention are used inthe high density recording systems with the shortest recordingwavelength of 1.0 μm or less, a P-V value (in terms of the opticalinterference type three-dimensional surface roughness) is preferably 50nm or less, more preferably 40 nm or less, to achieve the high output.That is, with the conventional acicular magnetic powder, when themagnetic recording media are produced to have a multiple layer-structurehaving the undercoat layer to decrease the thickness of the magneticlayer, the magnetic powder particles tend to penetrate in the undercoatlayer in comparison with the direct application of the magnetic layer onthe non-magnetic support. Therefore, the magnetic powder particles arenot aligned in parallel with the surface of the magnetic layer, so thatthe surface properties including surface smoothness tends todeteriorate. However, since the magnetic powder particles of the presentinvention have the spherical or ellipsoidal shape, they do notdeteriorate the surface properties in the course of the orientation. Inaddition, although the magnetic powder of the present invention consistsof very fine particles having an average particle size of 5 to 200 nm,it hardly agglomerates, and thus it has good dispersibility. As aresult, the magnetic powder of the present invention can improve thesurface smoothness of the magnetic layer and achieve the high output incooperation with the above-described high coercive force, even when theshortest recording wavelength is 1.0 μm or less.

Herein, the surface roughness is measured using a non-contact typesurface roughness meter TOPO-3D (manufactured by WYKO) to which anobject head (magnification of 40 times) is attached, at a measuringwavelength of 648.9 nm and a measuring area of 250 μm×250 μm with thecurvatures and cylindrical corrections. The surface roughness ismeasured 4 times at each measuring point and the measured values areaveraged to obtain the surface roughness (P-V) at each point, and thesurface roughness values at 10 measuring points are again averaged.

Since the magnetic recording medium should be in contact with themagnetic head with the medium being wound around the cylinder in thehelical scanning system, the strength of the magnetic recording mediumin the machine direction and the transverse direction should beoptimized to increase the head contact of the medium. Quite recently, inthe helical scanning system, the tip of the magnetic head is shaped tohave an acute angle so that the amount of indentation in the magneticlayer increases, and the system is designed so that the relative speedof the magnetic tape and the magnetic head is very high. Therefore, thedeterioration of the head contact leads to the deterioration of anenvelope. From such a viewpoint, to improve the head contact of themedium against the magnetic head, a ratio of a Young's modulus in thetransverse direction (Y_(TD)) to that (Y_(MD)) in the machine directionof the medium (Y_(TD)/Y_(MD)) is preferably from 1.0 to 1.7. Since theconventional magnetic powder particles have the needle-form shape, theyare oriented so that the major axes are in parallel with the plane ofthe magnetic layer by the mechanical orientation step when the magneticpaint is applied. In addition, they are oriented in the magnetic fieldin the machine direction to attain the high squareness. Thus, the majoraxes of the particles are further aligned in the machine direction.Therefore, the strength of the magnetic layer in the machine directionis inevitably stronger than that in the transverse direction, and thehead contact against the magnetic head, which is desired to beisotropic, deteriorates. In contrast, since the magnetic recording mediaof the present invention use the magnetic powder particles having thespherical or ellipsoidal shape, the magnetic powder particles are hardlymechanically oriented in the course of the application of the magneticpaint in comparison with the acicular magnetic powder, and they are lessoriented in parallel with the plane of the magnetic layer. As a result,the strength of the magnetic recording media in the transverse directioncan be increased. Thus, the above ratio (Y_(TD)/Y_(MD)) is preferablyfrom 1.2 to 1.6.

Herein, the Young's modulus is measured with 0.3% elongation at 25° C.,60% RH.

When the magnetic layer is made thin in the present invention, a leastone undercoat layer is preferably provided between the non-magneticsupport and the magnetic layer so that the good surface smoothnessresulting from the particle shape of the magnetic powder can be attainedreadily. Since the specific magnetic powder used in the presentinvention can provide the magnetic paint with good flowability so thatthe leveling of the applied paint is improved and thus the formedmagnetic layer has good surface smoothness. When the undercoat layerhaving the similar coating properties to those of the magnetic paint isprovided, the leveling of the applied magnetic paint is improved incomparison to the direct application of the magnetic paint to thenon-magnetic support, and also the influence of the surface conditionsof the non-magnetic support on the surface properties of the magneticlayer can be suppressed.

The undercoat layer may contain inorganic powder, a binder, a lubricant,carbon black, and so on. The inorganic powder is preferably anon-magnetic powder, but a magnetic powder may be used in specialapplications.

Examples of the non-magnetic powder include β-alumina having analphatization degree of at least 90%, β-alumina, γ-alumina, α-ironoxide, TiO₂ (rutile or anatase type), TiOx, cerium oxide, tin oxide,tungsten oxide, ZnO, ZrO₂, SiO₂, Cr₂O₃, goethite, corundum, siliconnitride, titanium carbide, magnesium oxide, boron nitride, molybdenumdisulfide, copper oxide, MgCO₃, CaCO₃, BaCO₃, SrCO₃, BaSO₄, siliconcarbide, titanium carbide, and mixtures thereof. Examples of themagnetic powder include γ-Fe₂O₃, cobalt-containing γ-Fe₂O₃, Fe alloys,CrO₂, barium ferrite, etc.

The inorganic powders may have spherical, acicular or platelet shapes.The particle size of the inorganic powder preferably do not exceed 0.5μm, since the inorganic powder having the too large particle sizedeteriorates the surface properties of the undercoat layer and in turninfluences the surface properties of the magnetic layer. When theparticle size of the inorganic powder is too small, the filling rate ofthe inorganic powder in the undercoat layer increases so that the volumeof vacancies which retain the lubricant decreases and also thecushioning effects deteriorate. Thus, the particle size of the inorganicpowder is preferably at least 0.05 μm. In the case of acicularnon-magnetic powder, it usually has a major (longer) axis length of 0.05to 0.5 μm and an acicular ratio of 3 to 20. In the case of sphericalnon-magnetic powder, it usually has a particle size of 0.05 to 0.3 μm.

The amount of the inorganic powder used is preferably from 60 to 90 wt.%, particularly from 70 to 80 wt. % for the same reasons as describedabove in connection with the particle size.

The binder used in the undercoat layer may be the resin as that used inthe formation of the magnetic layer, and is preferably the same kindresin as that contained in the magnetic layer. In particular, when thesame combination of the vinyl chloride resin and the polyurethane resinis used in the magnetic layer and the undercoat layer, the elasticitiesof the both layers are close so that the load from the magnetic head canbe scattered in the both layers.

The binder in the undercoat layer preferably has the same functionalgroup(s) as that of the binder in the magnetic layer. In particular, inthe combination of the vinyl chloride resin and the polyurethane resin,the resins in the undercoat layer and those in the magnetic layerpreferably have the same functional groups, since the adhesion betweenthe two layer is increased, and furthermore the exudation of thelubricant from the undercoat layer to the magnetic layer is facilitated.

The amount of the binder in the undercoat layer is preferably from 20 to45 wt. parts, particularly from 25 to 40 wt. parts, per 100 wt. parts ofthe inorganic powder.

Furthermore, it is preferable to use a thermally curing crosslinkingagent, which crosslinks the binder through the bonding of the functionalgroups of the binder, like in the case of the magnetic layer. The amountof the crosslinking agent is preferably from 15 to 70 wt. parts per 100wt. parts of the binder.

Also, the same lubricant as one used in the magnetic layer can be usedin the undercoat layer, but it is preferable to use the fatty acid esteronly, or the mixture of the fatty acid and the fatty acid ester havingthe increased ratio of the fatty acid ester, since the fatty acid isless exuded in the upper magnetic layer than the fatty acid ester. Theamount of the lubricant added to the undercoat layer is usually from 0.5to 12 wt. parts, preferably from 1 to 10 wt. parts, more preferably from2 to 10 wt. parts, per 100 wt. parts of the inorganic powder. The weightratio of the fatty acid to the fatty acid ester added to the undercoatlayer is preferably from 0:100 to 40:60, particularly from 0:100 to30:70.

To add the lubricant to the undercoat layer, the lubricant is added to apaint for the undercoat layer before, during or after mixing with akneader and the like, or the solution of the lubricant is applied orspray coated to the surface of the already formed undercoat layer.

As the carbon black used in the undercoat layer, a combination of carbonblack having a particle size of 0.01 to 0.03 μm and carbon black havingparticle size of 0.05 to 0.3 μm is preferably used. The former carbonblack is used to maintain the electrical conductivity and retain thevacancies which keep the lubricant like in the case of the magneticlayer, while the latter carbon black copes with both the increase of thefilm strength of the undercoat layer and the cushioning effects. Theamount of carbon black added to the undercoat layer in total ispreferably from 5 to 70 wt. parts, particularly from 15 to 40 wt. parts,per 100 wt. parts of the inorganic powder.

Examples of the carbon black having a particle size of 0.01 to 0.03 μminclude “BLACK PEARLS 800”, “Mogul-L”, “VULCAN XC-72”, “Regel 660R” (allavailable from Cabot); “Raven 1255” and “Conductex SC” (both availablefrom Columbian Carbon); etc. Examples of the carbon black havingparticle size of 0.05 to 0.3 μm include “BLACK PEARLS 130” and “Monarch120” (both available from Cabot); “Raven 450” and “Raven 410” (bothavailable from Columbian Carbon); “Termax Powder N-991” (available fromCANCARB); etc.

As the solvents used to prepare the paint for the undercoat layer or thelubricant solution in the formation of the undercoat layer, organicsolvents such as aromatic solvents, ketone solvents, ester solvents,alcohols, hexane, tetrahydrofuran, and so on may be used like in theformation of the magnetic layer.

The average thickness of the undercoat layer is preferably from 0.5 to10 μm, more preferably from 1 to 5 μm. The average thickness of theundercoat layer is preferably 1.7 to 200 times, more preferably 2 to 50times larger than the average thickness of the magnetic layer.

Herein, the average thickness of the magnetic layer or the undercoatlayer is obtained by cutting the magnetic recording medium with amicrotome, taking a transmission electron microscopic photograph of thecross section of the cut medium (magnification: 50,000 times), measuringthe thickness of the magnetic layer or the undercoat layer at ten pointswith an interval of 1 cm, repeating this measurement at five differentparts and averaging the fifty (10×5) measured values.

In the present invention, the non-magnetic support may be any one ofthose conventionally used in the magnetic recording media. Specificexamples of the support are plastic films of polyesters (e.g.polyethylene terephthalate, polyethylene naphthalate, etc.), polyolefin,cellulose triacetate, polycarbonate, polysulfone, polyamides (e.g.polyamide, polyimide, polyamideimide, aramide, aromatic polyamide,etc.), and the like. The non-magnetic support has a thickness of 2 to100 μm. Among the non-magnetic supports, the polyester film or thepolyamide film with the improved strength in the transverse direction ispreferably used, which has a Young's modulus of at least 5.0×10⁹ N/m²,preferably 6.0×10⁹ N/m² to 22.0 10⁹ N/m² in the transverse direction atan elongation of 0.3%, to improve the head contact with the magnetichead when the total thickness of the medium is reduced for the purposeof the high density recording.

It is preferable to use a non-magnetic support having different surfaceroughness on both surfaces when a back coat layer is formed on thesurface of the support opposite to the magnetic layer. The difference ofthe surface properties makes it easy to control the P-V value of themagnetic layer.

The non-magnetic support may have a resin layer on its surface toimprove the adhesion to the undercoat layer. Examples of the resin ofthe resin layer include polyester resins, polyurethane resins, etc.Among them, the resins, having functional groups such as —COOM, —SO₃M,—OSO₃M, —P═O(OM)₃, —O—P═O (OM)₂ (wherein M is a hydrogen atom, an alkalimetal or an amine group) are preferable, since they have good adhesionto the non-magnetic support and improve the adhesion to the undercoatlayer. The resin layer may contain an inorganic powder such as silica toprevent blocking. The thickness of the resin layer is preferably 0.1 μmor less, particularly from 0.01 to 0.08 μm.

When the non-magnetic support has the large anisotropy of shrinkagewhich is generated in a service atmosphere, in particular a hightemperature atmosphere, the followability deteriorates and thus thetracking errors tend to occur. Therefore, the non-magnetic supportpreferably has a thermal shrinkage of 1.5% or less in the machinedirection and 1.0% or less in the transverse direction at 105° C., 30minutes, that is, when the thermal shrinkage is measured by heating thesupport at 105° C. for 30 minutes and then cooling it. In detail, thethermal shrinkage is measured as follows:

Six samples each having a width of 10 mm and a length of 300 mm arecollected from the non-magnetic support in the machine direction or thetransverse direction and heated at 105° C. for 30 minutes in a hot airfollowed by cooling. The length of each sample is measured, and thethermal shrinkage is calculated according to the following equation:Thermal shrinkage (%)=[(Original length−Length after shrink)/Originallength]×100 Then the calculated thermal shrinkage values of six samplesare averaged.

In the application steps to form the undercoat layer and the magneticlayer on the non-magnetic support according to the present invention,any conventional application methods such as gravure coating, rollcoating, blade coating, extrusion coating, etc. may be used. Theapplication method of the undercoat layer and the magnetic layer maybethe sequential multiple layer coating method in which the magnetic paintof the magnetic layer is applied on the undercoat layer which has beenapplied on the non-magnetic support and dried, or the simultaneousmultiple layer coating method in which the undercoat layer and themagnetic layer are applied at the same time. In view of the leveling ofthe thin magnetic layer in the course of the application, thesimultaneous multiple layer coating method, which applies the paint forthe magnetic layer while the undercoat layer is still wet, is preferablyused. The present invention is particularly effective in thesimultaneous multiple layer coating method, since in the simultaneousmultiple layer coating method which applies the magnetic layer while theundercoat layer is still wet, the interface between the undercoat layerand the magnetic layer is disturbed and the magnetic powder particlestend to penetrate in the undercoat layer so that the surface propertiesof the magnetic layer tend to deteriorate.

The magnetic recording media of the present invention may have a backcoat layer-on the surface of the non-magnetic support opposite to themagnetic layer. Besides conductive carbon black, the back coat layer maycontain inorganic non-magnetic powder which is known as an abrasive todecrease the coefficient of friction and to increase the mechanicalstrength. Examples of such non-magnetic powder include α-Fe₂O₃, Fe₃O₄,TiO₂, graphite, CaO, SiO₂, Cr₂O₃, α-Al₂O₃, SiC, CaCO₃, BaSO₄, ZnO, MgO,boron nitride, TiC, ZnS, MgCO₃, SnO₃, etc. If desired, the back coat mayfurther contain lubricants such as higher fatty acids, fatty acidesters, silicone oils, etc.; dispersants such as surfactants; and otheradditives.

The binders of the back coat layer may be the same as those used in themagnetic layer. Among them, the combination of the cellulose resin andthe polyurethane is preferable.

The amount of the binder used is preferably from about 15 to 200 wt.parts per 100 wt. parts of the carbon black and the inorganicnon-magnetic powder. To cure the binder, a crosslinking agent such aspoyisocyante may be used in combination with the binder.

The average thickness of the back coat layer is preferably from about0.3 to 1.0 μm after calendering. When the thickness of the back coatlayer is too large, the total thickness of the magnetic recording mediumbecomes too large. When the thickness of the back coat layer is toosmall, the surface properties of the back coat layer deteriorate by theinfluence of the surface properties of the non-magnetic support so thatthe surface conditions of the back coat layer are transferred to themagnetic layer surface and thus the electromagnetic conversioncharacteristics, etc. may deteriorate.

In the production of the magnetic recording media of the presentinvention, the surface of the magnetic layer is preferably treated bycalendering with plastic rolls or metal rolls, for example, five metalrolls. The calendering can adjust the P-V value of the surface of themagnetic layer. In addition, the filling rate of the magnetic powder canbe increased to increase the residual magnetic flux density. Acalendering temperature is preferably at least 60° C., particularly from80 to 200° C. A linear pressure is preferably at least 115 kN/m,particularly from 150 to 400 kN/m, and a calendering speed is preferablyfrom 20 to 700 m/min. In particular, the above effects can be enhancedwhen the calendering is carried out at a temperature of at least 80° C.under a linear pressure of at least 190 kN/m.

In the production of the magnetic recording media of the presentinvention, the media are aged after the above calendering. The aging canpromote the curing of the coated film and improve the film strength. Theaging is preferably carried out at a temperature of 70° C. or less,since when the aging temperature is too high, the winding constrictionof the magnetic sheet becomes too high so that the surface roughness ofthe back coat layer is transferred to the magnetic layer and thus thesurface properties of the magnetic layer tend to deteriorate. To adjustthe wetness, the aging is preferably carried out under a humidity of 5to 60% RH.

Furthermore, it is preferable to abrade the surface of the magneticlayer after drying to remove dusts causing dropouts from the surface ofthe magnetic surface and also the vulnerable parts of the surface of themagnetic layer, and to adjust the surface properties of the magneticlayer. The abrading treatment may be carried out with a blade or anabrasive wheel. From the viewpoint of the productivity, the treatmentwith the abrasive wheel is preferable. The treatment with the abrasivewheel is described in, for example, JP-A-62-150519, JP-A-62-172532,JPA-A-2-23521, etc. As a material used to form the abrading part of thewheel, ceramics, super steel, sapphire, diamond, and the like may beexemplified. When the abrasive wheel is used, the peripheral speed ofthe wheel is preferably ±200% of the tape running speed (50 to 300m/min.), and an winding angle of the tape around the wheel is preferablyfrom 10 to 80 degrees.

EXAMPLES

The present invention will be illustrated by the following Examples, inwhich “parts” mean “parts by weight” unless otherwise indicated.

Preparation of Magnetic Powder (Examples 1-14 and Comparative Examples1-2) Example 1

Cobalt nitrate hexahydrate (0.419 mole) and iron(III) nitratenonahydrate (0.974 mole) were dissolved in water (1500 ml). Separately,sodium hydroxide (3.76 moles) was dissolved an water (1500 g). Thelatter solution of sodium hydroxide was added to the former solution ofthe nitrates and the mixture was stirred for 20 minutes to coprecipitateiron and cobalt.

The coprecipitated material was placed in an autoclave and heated at220° C. for 4 hours. After hydrothermal treatment, the precipitate waswashed with water to obtain spherical or ellipsoidal cobalt ferriteparticles having a particle size of 15 nm.

Then, the cobalt ferrite particles (20 g) were suspended in water (200g). To the suspension, samarium nitrate hexahydrate (0.00726 mole) wasadded and dissolved followed by stirring for 20 minutes. Furthermore, asolution of sodium hydroxide (0.02178 mole) in water (10 g) was added tothe suspension and stirred for 20 minutes.

The suspension was heated at 90° C. for 1 hour, washed with water andfiltrated. The filtrated material was spread in a vat and dried at 60°C. for 6 hours to remove moisture.

The resulting oxide was ground with a mortar, placed in a tubularelectric furnace and heated in a hydrogen stream at 500° C. for 1 hourto reduce the oxide. The reduced material was cooled to room temperaturewhile flowing the hydrogen gas and then the gas was switched to anitrogen gas containing 1000 ppm of oxygen. Thereafter, the temperaturewas raised to 100° C. and the material was stabilized for 6 hours in thesame oxygen-containing nitrogen gas stream. After cooling, the materialwas recovered in an air.

According to an X-ray fluorescent analysis, the obtainedsamarium-containing iron-cobalt magnetic powder contained 5.6 atomic %of samarium based on the total of the transition metals (iron andcobalt), and the atomic ratio of cobalt to iron was 29:71.

The obtained magnetic powder was observed with a transmission electronmicroscope (magnification: 300,000 times) The electron microscopicphotograph is shown in FIG. 1. The powder consisted of substantiallyspherical or ellipsoidal particles having a particle size of 15 nm.

The magnetic powder had a saturation magnetization of 21.6 μWb/g and acoercive force of 125.7 kA/m when measured with applying a magneticfield of 16 kOe.

Example 2

A samarium-containing iron-cobalt magnetic powder was produced in thesame manner as in Example 1 except that cobalt ferrite particles wereprepared by changing the hydrothermal treatment conditions to 180° C.and 4 hours to obtain cobalt ferrite particles having a particle size of10 nm.

According to an X-ray fluorescent analysis, the obtainedsamarium-containing iron-cobalt magnetic powder contained 5.7 atomic %of samarium based on the total of the transition metals (iron andcobalt), and the atomic ratio of cobalt to iron was 30:70.

The obtained magnetic powder was observed with a transmission electronmicroscope. The powder consisted of substantially spherical orellipsoidal particles having a particle size of 10 nm.

The magnetic powder had a saturation magnetization of 20.6 μWb/g and acoercive force of 136.1 kA/m when measured with applying a magneticfield of 16 kOe.

Example 3

A samarium-containing iron-cobalt magnetic powder was produced in thesame manner as in Example 1 except that cobalt ferrite particles wereprepared by changing the hydrothermal treatment conditions to 260° C.and 4 hours to obtain cobalt ferrite particles having a particle size of20 nm.

According to an X-ray fluorescent analysis, the obtainedsamarium-containing iron-cobalt magnetic powder contained 5.4 atomic %of samarium based on the total of the transition metals (iron andcobalt), and the atomic ratio of cobalt to iron was 29:71.

The obtained magnetic powder was observed with a transmission electronmicroscope. The powder consisted of substantially spherical orellipsoidal particles having a particle size of 20 nm.

The magnetic powder had a saturation magnetization of 21.9 μWb/g and acoercive force of 123.3 kA/m when measured with applying a magneticfield of 16 kOe.

Example 4

A samarium-containing iron-cobalt magnetic powder was produced in thesame manner as in Example 1 except that the cobalt ferrite particlesprepared in Example 1 (10 g) was suspended in water (200 g), samariumnitrate hexahydrate (0.0118 mole) was added to and dissolved in thesuspension followed by stirring for 20 minutes, and then, a solution ofsodium hydroxide (0.0354 mole) in water (10 g) was added to thesuspension and stirred for 20 minutes.

According to an X-ray fluorescent analysis, the obtainedsamarium-containing iron-cobalt magnetic powder contained 9.4 atomic %of samarium based on the total of the transition metals (iron andcobalt), and the atomic ratio of cobalt to iron was 28:72.

The obtained magnetic powder was observed with a transmission electronmicroscope. The powder consisted of substantially spherical orellipsoidal particles having a particle size of 15 μm.

The magnetic powder had a saturation magnetization of 20.3 μWb/g and acoercive force of 138.5 kA/m when measured with applying a magneticfield of 16 kOe.

Example 5

A neodymium-containing iron-cobalt magnetic powder was produced in thesame manner as in Example 1 except that the cobalt ferrite particlesprepared in Example 1 (10 g) was suspended in water (200 g), neodymiumnitrate hexahydrate (0.00726 mole) was added to and dissolved in thesuspension followed by stirring for 20 minutes, and then, a solution ofsodium hydroxide (0.02178 mole) in water (10 g) was added to thesuspension and stirred for 20 minutes.

According to an X-ray fluorescent analysis, the obtainedneodymium-containing iron-cobalt magnetic powder contained 5.5 atomic %of neodymium based on the total of the transition metals (iron andcobalt), and the atomic ratio of cobalt to iron was 29:71.

The obtained magnetic powder was observed with a transmission electronmicroscope. The powder consisted of substantially spherical orellipsoidal particles having a particle size of 15 nm.

The magnetic powder had a saturation magnetization of 21.5 μWb/g and acoercive force of 124.1 kA/m when measured with applying a magneticfield of 16 kOe.

Example 6

An yttrium-containing iron-cobalt magnetic powder was produced in thesame manner as in Example 1 except that the cobalt ferrite particlesprepared in Example 1 (10 g) was suspended in water (200 g), yttriumnitrate hexahydrate (0.00726 mole) was added to and dissolved in thesuspension followed by stirring for 20 minutes, and furthermore, asolution of sodium hydroxide (0.02178 mole) in water (10 g) was added tothe suspension and stirred for 20 minutes.

According to an X-ray fluorescent analysis, the obtainedyttrium-containing iron-cobalt magnetic powder contained 5.6 atomic % ofyttrium based on the total of the transition metals (iron and cobalt),and the atomic ratio of cobalt to iron was 28:72.

The obtained magnetic powder was observed with a transmission electronmicroscope. The powder consisted of substantially spherical orellipsoidal particles having a particle size of 15 nm.

The magnetic powder had a saturation magnetization of 21.9 μWb/g and acoercive force of 120.2 kA/m when measured with applying a magneticfield of 16 kOe.

Example 7

A samarium/yttrium-containing iron-cobalt magnetic powder was producedin the same manner as in Example 1 except that samarium nitratehexahydrate (0.00508 mole) and yttrium nitrate hexahydrate (0.00218mole) were added to and dissolved in the suspension of cobalt ferriteparticles prepared in Example 1 (10 g) in water (200 g).

According to an X-ray fluorescent analysis, the obtainedsamarium/yttrium-containing iron-cobalt magnetic powder contained 3.8atomic % of samarium and 1.6 atomic % of yttrium based on the total ofthe transition metals (iron and cobalt), and the atomic ratio of cobaltto iron was 28:72.

The obtained magnetic powder was observed with a transmission electronmicroscope. The powder consisted of substantially spherical orellipsoidal particles having a particle size of 15 nm.

The magnetic powder had a saturation magnetization of 21.5 μWb/g and acoercive force of 124.9 kA/m when measured with applying a magneticfield of 16 kOe.

Example 8

Spherical or ellipsoidal cobalt ferrite particles were prepared in thesame manner as in Example 1 except that the amounts of cobalt nitratehexahydrate and iron(III) nitrate nonahydrate were changed to 0.182 moleand 1.211 mole, respectively, and the amount of sodium hydroxide waschanged to 4.00 moles. The cobalt ferrite particles had a particle sizeof 18 nm.

Then, a samarium-containing iron-cobalt magnetic powder was produced inthe same manner as in Example 1 except that the above cobalt ferriteparticles (10 g) was suspended in water (200 g), samarium nitratehexahydrate (0.00726 mole) was added to and dissolved in the suspensionfollowed by stirring for 20 minutes, and then, a solution of sodiumhydroxide (0.02178 mole) in water (10 g) was added to the suspension andstirred for 20 minutes.

According to an X-ray fluorescent analysis, the obtainedsamarium-containing iron-cobalt magnetic powder contained 5.6 atomic %of samarium based on the total of the transition metals (iron andcobalt), and the atomic ratio of cobalt to iron was 13:87.

The obtained magnetic powder was observed with a transmission electronmicroscope. The powder consisted of substantially spherical orellipsoidal particles having a particle size of 18 nm.

The magnetic powder had a saturation magnetization of 21.0 μWb/g and acoercive force of 120.2 kA/m when measured with applying a magneticfield of 16 kOe.

Example 9

A samarium-containing iron-cobalt magnetic powder was produced in thesame manner as in Example 1 except that the cobalt ferrite particlesprepared in Example 1 (10 g) was suspended in water (200 g), samariumnitrate hexahydrate (0.00223 mole) was added to and dissolved in thesuspension followed by stirring for 20 minutes, and furthermore, asolution of sodium hydroxide (0.0067 mole) in water (10 g) was added tothe suspension and stirred for 20 minutes.

According to an X-ray fluorescent analysis, the obtainedsamarium-containing iron-cobalt magnetic powder contained 1.7 atomic %of samarium based on the total of the transition metals (iron andcobalt), and the atomic ratio of cobalt to iron was 29:71.

The obtained magnetic powder was observed with a transmission electronmicroscope. The powder consisted of substantially spherical orellipsoidal particles having a particle size of 15 nm.

The magnetic powder had a saturation magnetization of 22.7 μWb/g and acoercive force of 102.7 kA/m when measured with applying a magneticfield of 16 kOe.

Example 10

Cobalt ferrite particles were prepared in the same manner as in Example1 except that the amounts of cobalt nitrate and iron(III) nitrate wereused in the same amounts but the amount of sodium hydroxide was changedfrom 3.76 moles to 5.64 moles, and the hydrothermal treatment conditionswere changed to 280° C. and 4 hours. The cobalt ferrite particles had aparticle size of 35 nm.

Then, a samarium-containing iron-cobalt magnetic powder was produced inthe same manner as in Example 1 except that the above cobalt ferriteparticles (10 g) was suspended in water (200 g).

According to an X-ray fluorescent analysis, the obtainedsamarium-containing iron-cobalt magnetic powder contained 5.1 atomic %of samarium based on the total of the transition metals (iron andcobalt), and the atomic ratio of cobalt to iron was 28:72.

The obtained magnetic powder was observed with a transmission electronmicroscope. The powder consisted of substantially spherical orellipsoidal particles having a particle size of 35 nm.

The magnetic powder had a saturation magnetization of 22.4 μWb/g and acoercive force of 104.2 kA/m when measured with applying a magneticfield of 16 kOe.

Example 11

Iron(II) sulfate heptahydrate (0.419 mole) and iron(III) nitratenonahydrate. (0.974 mole) were dissolved in water (1500 g). Separately,sodium hydroxide (3.76 moles) was dissolved in water (1500 g). Then, theaqueous solution of sodium hydroxide was added to the solution of theiron salts and stirred for 20 minutes to grow magnetite particles.

The magnetite particles were placed in an autoclave and heated at 180°C. for 4 hours. After hydrothermal treatment, the particles were washedwith water to obtain spherical or ellipsoidal magnetite particles havinga particle size of 20 nm.

Then, the magnetite particles (10 g) were suspended in water (200 g). Inthe same manner as in Example 6, yttrium nitrate hexahydrate (0.00726mole) was added to and dissolved in this suspension followed by stirringfor 20 minutes. Furthermore, a solution of sodium hydroxide (0.02178mole) in water (10 g) was added to the suspension and stirred for 20minutes.

The resulting oxide was ground with a mortar, placed in a tubularelectric furnace and heated in a hydrogen stream at 450° C. for 1 hourto reduce the oxide. The reduced material was cooled to room temperaturewhile flowing the hydrogen gas, and then the gas was switched to anitrogen gas containing 1000 ppm of oxygen. Thereafter, the temperaturewas raised to 100° C. and the material was stabilized for 6 hours in thesame oxygen-containing nitrogen gas. After cooling, theyttrium-containing iron magnetic powder was recovered in an air.

According to an X-ray fluorescent analysis, the obtainedyttrium-containing iron magnetic powder contained 5.5 atomic % ofyttrium based on the iron.

The obtained magnetic powder was observed with a transmission electronmicroscope. The powder consisted of spherical or ellipsoidal particleshaving a particle size of 15 nm.

The magnetic powder had a saturation magnetization of 19.1 μWb/g and acoercive force of. 102.8 kA/m when measured with applying a magneticfield of 16 kOe.

Example 12

In the process of Example 11, after the reduction in the hydrogen gasstream at 450° C. for 1 hour, the reduced particles were cooled to 150°C. while flowing the hydrogen gas, and then nitrided in an ammonia gasat 150° C. for 30 hours. Thereafter, the temperature was further loweredto 100° C. After switching the gas to a nitrogen gas containing 1000 ppmof oxygen, the particles were stabilized for 6 hours. Then, theparticles were cooled to room temperature while flowing the sameoxygen-containing nitrogen gas and recovered in the air.

According to the X-ray fluorescent analysis, the obtained magneticpowder contained 5.5 atomic % of yttrium based on iron.

The obtained magnetic powder was observed with a transmission electronmicroscope. FIG. 2 shows the transmission electron microscopicphotograph of this magnetic powder (magnification: 300,000 times). Thepowder consisted of substantially spherical or ellipsoidal having anaverage particle size of 15 nm.

The X-ray diffraction analysis of the magnetic powder confirmed that ithad a mixed phase of iron nitride having the structure of Fe₁₆N₂ and theα-Fe phase.

The magnetic powder had a saturation magnetization of 15.1 μWb/g (120.2emu/g) and a coercive force of 210.9 kA/m (2650 Oe) when measured withapplying a magnetic field of 16 kOe.

Comparative Example 1

The cobalt ferrite particles synthesized in Example 1 were reduced underthe same conditions as those in Example 1 except that no rare earthelement was added, and then stabilized to obtain an iron-cobalt magneticpowder.

This magnetic powder was observed with a transmission electronmicroscope. The particles were apparently agglomerated by sintering, andthe particle sizes widely distributed from about 20 nm to about 300 nm(0.3 μm).

The magnetic powder had a saturation magnetization of 24.4 μWb/g and acoercive force of 9.5 kA/m when measured with applying a magnetic fieldof 16 kOe.

Comparative Example 2

The cobalt ferrite particles synthesized in Example 8 were reduced underthe same conditions as those in Example 1 except that no rare earthelement was added, and then stabilized to obtain an iron-cobalt magneticpowder.

This magnetic powder was observed with a transmission electronmicroscope. The particles were apparently agglomerated by sintering, andthe particle sizes widely distributed from about 20 nm to about 500 nm(0.5 μm).

The magnetic powder had a saturation magnetization of 23.9 μWb/g and acoercive force of 7.2 kA/m when measured with applying a magnetic fieldof 16 kOe.

Example 13

The magnetite particles prepared in Example 11 (10 g) was suspended inwater (200 g). Then, samarium nitrate hexahydrate (0.00726 mole) wasadded to and dissolved in the suspension followed by stirring for 20minutes, and furthermore, a solution of sodium hydroxide (0.02178 mole)in water (10 g) was added to the suspension and stirred for 20 minutes.Thereafter, the oxide was ground with a mortar, and reduced andstabilized in the same manners as those in Example 11 to obtain asamarium-containing iron magnetic powder.

According to an X-ray fluorescent analysis, the obtainedsamarium-containing iron magnetic powder contained 5.6 atomic % ofsamarium based on the iron.

The obtained magnetic powder was observed with a transmission electronmicroscope. The powder consisted of substantially spherical orellipsoidal particles having a particle size of 15 nm.

The magnetic powder had a saturation magnetization of 18.8 μWb/g and acoercive force of 112.3 kA/m when measured with applying a magneticfield of 16 kOe.

Example 14

In the process of Example 13, after the reduction in the hydrogen gasstream at 450° C. for 1 hour, the reduced particles were nitrided underthe same conditions as those in Example 12 and stabilized. Then theparticles were recovered in the air.

According to the X-ray fluorescent analysis, the obtained magneticpowder contained 5.6 atomic % of samarium based on iron.

The obtained magnetic powder was observed with a transmission electronmicroscope. The powder consisted of substantially spherical orellipsoidal having an average particle size of 15 nm.

The X-ray diffraction analysis of the magnetic powder confirmed that ithad a mixed phase of iron nitride having the structure of Fe₁₆N₂ and theα-Fe phase.

The magnetic powder had a saturation magnetization of 15.5 μWb/g (123.4emu/g) and a coercive force of 208.5 kA/m (2620 Oe) when measured withapplying a magnetic field of 16 kOe.

Table 1 summarizes the contents of rare earth metal elements based onthe transition metal(s) (iron+cobalt, or iron), the cobalt to ironratios, the shapes and sizes of the particles and the magneticcharacteristics of the magnetic powders prepared in Examples 1-14 andComparative Examples 1-2.

TABLE 1 Rare earth element content based on transition metals Co/FeParticle Coercive Saturation Ex. Magnetic (Atomic %) atomic Particlesize force magnetization No. powder Sm Nd Y ratio shape (nm) (kA/m)(μWb/g)  1 Sm—Fe—Co 5.6 — — 29/71 Sphere or 15 125.7 21.6 ellipsoid  2Sm—Fe—Co 5.7 — — 30/70 Sphere or 10 136.1 20.6 ellipsoid  3 Sm—Fe—Co 5.4— — 29/71 Sphere or 20 123.3 21.9 ellipsoid  4 Sm—Fe—Co 9.4 — — 28/72Sphere or 15 138.5 20.3 ellipsoid  5 Nd—Fe—Co — 5.5 29/71 Sphere or 15124.1 21.5 ellipsoid  6 Y—Fe—Co — — 5.6 28/72 Sphere or 15 120.2 21.9ellipsoid  7 Sm—Y—Fe—Co 3.8 — 1.6 28/72 Sphere or 15 124.9 21.5ellipsoid  8 Sm—Fe—Co 5.6 — — 13/87 Sphere or 18 120.2 21.0 ellipsoid  9Sm—Fe—Co 1.7 — — 29/71 Sphere or 15 102.7 22.7 ellipsoid 10 Sm—Fe—Co 5.1— — 28/72 Sphere or 35 104.2 22.4 ellipsoid 11 Y—Fe — — 5.5  0/100Sphere or 15 102.8 19.1 ellipsoid 12 Y—N—Fe — — 5.5  0/100 Sphere or 15210.9 15.1 ellipsoid 13 Sm—Fe 5.6 — —  0/100 Sphere or 15 112.3 18.8ellipsoid 14 Sm—N—Fe 5.6 — —  0/100 Sphere or 15 208.5 15.5 ellipsoid C.1 Fe—Co — — — 29/71 Agglomerated 20-300 9.5 24.4 C. 2 Fe—Co — — — 29/71Agglomerated 20-300 7.2 23.9

Example 15

Magnetite particles were prepared in the same manner as in Example 11except that the hydrothermal treating temperature in the autoclave waschanged from 180° C. to 200° C. The resulting magnetite particles hadthe shape close to a sphere and an average particle size of 25 nm.

The magnetic particles (10 g) was dispersed in water (500 cc) for 30minutes using an ultrasonic disperser. Then, yttrium nitrate (2.5 g) wasadded to and dissolved in the dispersion followed by stirring for 30minutes. Separately, sodium hydroxide (0.8 g) was dissolved in water(100 cc). Then, the solution of sodium hydroxide was dropwise added tothe suspension of the magnetic particles over about 30 minutes followedby stirring for 1 hour. With this treatment, yttrium hydroxide wasdeposited on the surfaces of the magnetite particles.

The magnetite particles carrying the deposited yttrium hydroxide werewashed with water, recovered by filtration and then dried at 90° C. toobtain magnetite particles carrying yttrium hydroxide deposited on theirsurfaces.

Those magnetite particles were reduced by heating in a hydrogen gasstream at 450° C. for 2 hours to obtain an yttrium-iron magnetic powder.Then, the temperature was lowered to 150° C. over about 1 hour whileflowing the hydrogen gas. When the temperature reached 150° C., thehydrogen gas was switched to an ammonia gas, and the magnetite particleswere nitrided with ammonia for 30 hours while keeping the temperature at150° C. Thereafter, the temperature was lowered from 150° C. to 90° C.while flowing the ammonia gas. When the temperature reached 90° C., theammonia gas was switched to a nitrogen gas containing oxygen and theparticles were stabilized for 2 hours.

The temperature was then lowered from 90° C. to 40° C. while flowing theoxygen-containing nitrogen gas, and the particles were maintained at 40°C. for 10 hours and recovered in the air.

According to the X-ray fluorescent analysis, the obtained yttrium-ironnitride magnetic powder contained 5.3 atomic % of yttrium and 10.8atomic % of nitrogen based on iron.

FIG. 3 shows the X-ray diffraction pattern of this magnetic powder, inwhich the diffraction peaks assigned to Fe₁₆N₂ and α-Fe are observed.These peaks confirmed that the powder had a mixed phase of iron nitridehaving the structure of Fe₁₆N₂ and the α-Fe phase.

The obtained magnetic powder (300 particles) was observed with a highresolution transmission electron microscope at a magnification of200,000 times. FIG. 4 shows the transmission electron microscopicphotograph of this magnetic powder. The particles had a shape close to asphere and an average particle size of 20 nm.

The powder had a BET specific surface of 53.2 m²/g.

The magnetic powder had a saturation magnetization of 17.0 μWb/g (135.2Am²/kg) and a coercive force of 226.9 kA/m when measured with applying amagnetic field of 16 kOe. After maintaining the magnetic powder at 60°C., 90% RH for one week, the magnetic powder had a saturationmagnetization of 14.8 μWb/g (118.2 Am²/kg), which means that apreservation rate of saturation magnetization was 87.4%.

On the other hand, a magnetic powder having spherical or ellipsoidalshape consisting of the α-Fe phase containing no Fe₁₆N₂ phase wasproduced in the same manner as in Example 15 except that no nitridingtreatment was carried out. This magnetic powder had a preservation rateof saturation magnetization was 71.0%.

The magnetic powder of Example 15 had a higher preservation rate ofsaturation magnetization than the magnetic powder which was notnitrided, and thus good storage stability.

Example 16

A yttrium-iron nitride magnetic powder was produced in the same manneras in Example 15 except that the same magnetite particles as those usedin Example 11 was used as the raw material.

According to the X-ray fluorescent analysis, the obtained yttrium-ironnitride magnetic powder contained 5.5 atomic % of yttrium and 11.9atomic % of nitrogen based on iron.

The X-ray diffraction pattern of this magnetic powder showed the profilecorresponding to the Fe₁₆N₂ phase.

The obtained magnetic powder (300 particles) was observed with a highresolution transmission electron microscope at a magnification of200,000 times. The particles had spherical or ellipsoidal shapes and anaverage particle size of 17 nm.

The powder had a BET specific surface of 60.1 m²/g.

The magnetic powder had a saturation magnetization of 16.4 μWb/g (130.5Am²/kg) and a coercive force of 211.0 kA/m when measured with applying amagnetic field of 16 kOe. After maintaining the magnetic powder at 60°C., 90% RH for one week, the magnetic powder had a saturationmagnetization of 13.4 μWb/g (106.9 Am²/kg), which means that apreservation rate of saturation magnetization was 81.9%.

Example 17

Magnetite particles having an average particle size of 30 nm wereproduced in the same manner as in Example 11 except that thehydrothermal treatment temperature was changed from 180° C. to 220° C.

A yttrium-iron nitride magnetic powder was produced in the same manneras in Example 15 except that the magnetite particles produced in theprevious step were used.

According to the X-ray fluorescent analysis, the obtained yttrium-ironnitride magnetic powder contained 4.8 atomic % of yttrium and 10.1atomic % of nitrogen based on iron.

The X-ray diffraction pattern of this magnetic powder showed the profilecorresponding to the Fe₁₆N₂ phase.

The obtained magnetic powder (300 particles) was observed with a highresolution transmission electron microscope at a magnification of200,000 times. FIG. 5 shows the transmission electron microscopicphotograph of this magnetic powder. The particles had spherical orellipsoidal shapes and an average particle size of 27 nm.

The powder had a BET specific surface of 42.0 m²/g.

The magnetic powder had a saturation magnetization of 19.5 μWb/g (155.1Am²/kg) and a coercive force of 235.4 kA/m when measured with applying amagnetic field of 16 kOe. After maintaining the magnetic powder at 60°C., 90% RH for one week, the magnetic powder had a saturationmagnetization of 17.6 μWb/g (140.1 Am²/kg), which means that apreservation rate of saturation magnetization was 90.3%.

Production of Magnetic Tape (Examples 18-27 and Comparative Examples3-6) Example 18

The following components for an undercoat layer were kneaded with akneader and dispersed with a sand mill in a residence time of 60minutes. To the mixture, a polyisocyanate (6 parts) was added, stirredand then filtrated to obtain an undercoat paint.

Separately, the following components (1) for a magnetic paint werekneaded with a kneader and dispersed with a sand mill in a residencetime of 45 minutes. To this mixture, the components (2) for the magneticpaint were added, stirred and filtrated to obtain a magnetic paint.

<Components of undercoat paint> parts Titanium oxide powder (av.particle size: 0.035 μm) 70 Titanium oxide powder (av. particle size:0.1 μm) 10 Carbon black (av. particle size: 0.075 μm) 20 Vinyl chloridecopolymer 10 (SO₃Na groups: 0.7 × 10⁻⁴ eq./g) Polyester polyurethaneresin 5 (SO₃Na groups: 1.0 × 10⁻⁴ eq./g) Methyl ethyl ketone 130 Toluene80 Myristic acid 1 Butyl stearate 1.5 Cyclohexanone 65

<Magnetic paint components (1)> parts Samarium-cont. iron-cobaltmagnetic powder 100 according to Example 1 (Coercive force: 125.7 kA/m;Saturation magnetization: 21.6 μWb/g; Av. particle size: 15 nm; Sphereor ellipsoid) Vinyl chloride-hydroxypropyl acrylate copolymer 8 (SO₃Nagroups: 0.7 × 10⁻⁴ eq./g) Polyester polyurethane resin 4 (SO₃Na groups:1.0 × 10⁻⁴ eq./g) α-Alumina (av. particle size: 0.4 μm) 10 Carbon black(av. particle size: 100 nm) 1.5 Myristic acid 1.5 Methyl ethyl ketone133 Toluene 100

<Magnetic paint components (2)> parts Stearic acid 1.5 Polyisocyanate 4Cyclohexanone 133 Toluene 33

The undercoat paint was applied on a polyethylene terephthalate film(Degrees of thermal shrinkage of 0.8% and 0.6% in the machine andtransverse directions, respectively after heating at 105° C. for 30minutes) with the thickness of 6.0 μm as a non-magnetic support to forman undercoat layer having a thickness of 2 μm after drying andcalendering. On the undercoat layer, the magnetic paint was appliedwhile applying a magnetic field of 0.3 T along the machine direction sothat the magnetic layer had a thickness of 0.13 μm after drying andcalendering, and then dried.

Next, on the surface of the non-magnetic support opposite to the surfaceon which the undercoat layer and the magnetic layer were formed, a backcoat paint was applied so that the back coat layer had a thickness of0.7 μm after drying and calendering, and dried. The back coat paint wasprepared by dispersing the following components with a sand mill in aresidence time of 45 minutes, adding a polyisocyanate (8.5 parts) to themixture and then stirring and filtrating the mixture.

<Components of back coat paint> parts Carbon black (av. particle size:25 nm) 40.5 Carbon black (av. particle size: 370 nm) 0.5 Barium sulfate4.05 Nitrocellulose 28 Polyurethane resin (containing SO₃Na groups) 20Cyclohexanone 100 Toluene 100 Methyl ethyl ketone 100

The produced magnetic sheet was planish finished with five-stagecalendering (at 70° C. under a linear pressure of 147 kN/m) and aged at60° C., 40% RH for 48 hours with winding the sheet around a sheet core.Then, the sheet was slit at a width of 3.8 mm, and the surface of themagnetic layer of the obtained tape was abraded with a ceramic wheel (arotation speed of +150% and a winding angle of 30 degrees) whiletraveling the tape at a rate of 100 m/min. Thus, a magnetic tape havinga length of 125 m was obtained. The magnetic tape was installed in acartridge and used as a tape for a computer.

Example 19

A magnetic tape was produced in the same manner as in Example 18 exceptthat the thickness of the magnetic layer after drying and calenderingwas changed to 0.25 μm.

Example 20

A magnetic tape was produced in the same manner as in Example 18 exceptthat the thickness of the magnetic layer after drying and calenderingwas changed to 0.08 μm.

Example 21

A magnetic tape was produced in the same manner as in Example 18 exceptthat the yttrium containing iron magnetic powder of Example 11 (coerciveforce: 102.8 kA/m, saturation magnetization: 19.1 μWb/g, averageparticle size: 15 nm, particle shape: sphere or ellipsoid) was used as amagnetic powder, and the thickness of the magnetic layer after dryingand calendering was changed to 0.17 μm.

Example 22

A magnetic tape was produced in the same manner as in Example 18 exceptthat the thickness of the magnetic layer after drying and calenderingwas changed to 0.10 μm.

Example 23

A magnetic tape was produced in the same manner as in Example 18 exceptthat the thickness of the magnetic layer after drying and calenderingwas changed to 0.06 μm.

Example 24

A magnetic tape was produced in the same manner as in Example 18 exceptthat the yttrium-containing iron nitride magnetic powder of Example 12(coercive force: 1210.9 kA/m, saturation magnetization: 15.1 μWb/g,average particle size: 15 nm, particle shape: sphere or ellipsoid) wasused as a magnetic powder, and the thickness of the magnetic layer afterdrying and calendering was changed to 0.14 μm.

Example 25

A magnetic tape was produced in the same manner as in Example 18 exceptthat the samarium-containing iron nitride magnetic powder of Example 14(coercive force: 208.5 kA/m, saturation magnetization: 15.5 μWb/g,average particle size: 15 nm, particle shape: sphere or ellipsoid) wasused as a magnetic powder, and the thickness of the magnetic layer afterdrying and calendering was changed to 0.13 μm.

Example 26

A magnetic tape was produced in the same manner as in Example 24 exceptthat the magnetic paint was applied on the undercoat layer without theorientation with the magnetic field so that the thickness of themagnetic layer after drying and calendering was 0.12 μm.

In Examples 18 to 25, the coated magnetic paint was subjected to theorientation treatment in the machine direction with the magnetic fieldto achieve the high squareness in the machine direction, while inExample 26, no orientation treatment with the magnetic field was carriedout so that the same level of squareness could be attained in anydirections including the machine and perpendicular directions.

In general, since the conventional magnetic powder particles have anacicular shape, they are oriented in the machine direction to someextent in the absence of the orientation treatment with the magneticfield. However, since the magnetic powder particles of the presentinvention have a spherical or ellipsoidal shape, they are lessinfluenced by the mechanical orientation and thus the same level ofsquareness can be easily attained in any directions.

Example 27

A magnetic tape was produced in the same manner as in Example 24 exceptthat the magnetic paint was applied on the undercoat layer whileapplying a magnetic field of 0.3 T in the direction perpendicular to thecoated magnetic paint layer so that the thickness of the magnetic layerafter drying and calendering was 0.17 μm.

In Examples 18 to 25, the coated magnetic paint was subjected to theorientation treatment in the machine direction with the magnetic fieldto achieve the high squareness in the machine direction, and in Example26, the magnetic paint was applied on the undercoat layer without theorientation with the magnetic field so that the same level of squarenesscould be attained in any directions including the machine andperpendicular directions, while in Example 27, the magnetic field wasapplied in the direction perpendicular to the magnetic layer so that thehigh squareness could be attained in the direction perpendicular to themagnetic layer.

In general, since the conventional magnetic powder particles have anacicular shape, they are aligned on the surface of the undercoat layerin the direction perpendicular to the undercoat layer when the magneticfiled is applied in the direction perpendicular to the layer. As aresult, the surface of the magnetic layer is severely deteriorated.However, since the magnetic powder particles of the present inventionhave a spherical or ellipsoidal shape, they have substantially no shapeanisotropy and thus the produced recording media have the surfaceproperties comparable with the magnetic media which are oriented in themachine direction even when the magnetic powder particles are orientedin the direction perpendicular to the undercoat layer.

Comparative Example 3

A magnetic tape was produced in the same manner as in Example 15 exceptthat an acicular iron-cobalt alloy magnetic powder (Co: 24.6 atomic %based on iron; coercive force: 189.4 kA/m, saturation magnetization:18.3 μWb/g, average major axis length: 150 nm, acicular ratio: 5) wasused as a magnetic powder in the magnetic paint and the thickness of themagnetic layer after drying and calendering was changed to 0.50 μm.

Comparative Example 4

A magnetic tape was produced in the same manner as in ComparativeExample 3 except that the thickness of the magnetic layer after dryingand calendering was changed to 0.35 μm.

Comparative Example 5

A magnetic tape was produced in the same manner as in ComparativeExample 3 except that the thickness of the magnetic layer after dryingand calendering was changed to 0.20 μm.

Comparative Example 6

A magnetic tape was produced in the same manner as in Example 26 exceptthat the iron-cobalt magnetic powder of Comparative Example 1 (coerciveforce: 9.5 kA/m, saturation magnetization: 24.4 μWb/g, average particlesize: 20-300 nm) was used as a magnetic powder in the magnetic paint andthe thickness of the magnetic layer after drying and calendering waschanged to 1.1 μm.

With the magnetic tapes produced in Examples 18-27 and ComparativeExamples 3-6, a coercive force (Hc), a saturated magnetic flux density(Bm), a squareness (Br/Bm) and an anisotropic magnetic fielddistribution (Ha) were measured as the magnetic properties.

The anisotropic magnetic field distribution was expressed by a valueobtained by dividing a magnetic field corresponding to a half-widthvalue of a differential curve in the second quadrant of the hysteresiscurve (demagnetization curve) of the tape by the coercive force of thetape. That is, as the coercive force distribution of the magnetic powderis narrower or the dispersion and orientation of the magnetic powder inthe tape is better, Ha is smaller. When the coercive force is the same,the smaller Ha leads to the better recording characteristics inparticular in the short wavelength range.

As one of the electromagnetic conversion characteristics, a block errorrate (BER) was measured by recording random data signals of a shortestrecording wavelength of 0.33 μm with a DDS drive (C1554A manufactured byHewlett-Packard) and measuring a block error rate with a block errormeasuring apparatus.

The results are summarized in Table 2 together with the thickness of themagnetic layer of each magnetic tape.

TABLE 2 Thickness Anisotropic of Coercive magnetic magnetic forceSaturation Squareness Squareness field Ex. Magnetic layer (MD)magnetization (MD) (TD) distribution No. powder (μm) (kA/m) (T) (Br/Bm)(Br/Bm) (Ha) BER 18 Sm—Fe—Co 0.13 138.3 0.408 0.82 — 0.55 0.9 × E−03 19Sm—Fe—Co 0.25 133.7 0.419 0.84 — 0.53 1.0 × E−03 20 Sm—Fe—Co 0.08 140.10.388 0.82 — 0.57 1.3 × E−03 21 Sm—Fe—Co 0.17 120.5 0.397 0.88 — 0.571.3 × E−03 22 Sm—Fe—Co 0.10 118.8 0.391 0.87 — 0.56 0.9 × E−03 23Sm—Fe—Co 0.06 117.1 0.379 0.85 — 0.58 1.5 × E−03 24 Y—N—Fe 0.14 235.30.320 0.83 — 0.44 0.3 × E−03 25 Sm—N—Fe 0.13 232.1 0.325 0.85 — 0.48 0.4× E−03 26 Y—N—Fe 0.12 213.2 0.328 0.63 0.62  0.50 2.5 × E−03 27 Y—N—Fe0.17 137.6 0.311 0.41 0.77* 0.57 0.9 × E−03 C. 3 Acicular 0.50 183.80.395 0.83 — 0.61 1.6 × E−03 Fe—Co C. 4 Acicular 0.35 183.0 0.373 0.82 —0.61 3.8 × E−03 Fe—Co C. 5 Acicular 0.20 179.9 0.352 0.80 — 0.65 7.0 ×E−03 Fe—Co C. 6 Fe—Co 0.9 22.7 0.316 0.60 — 1.0<   1 × E−01< Note:*Squareness in the direction perpendicular after the diamagnetic fieldcorrection.

From the results in Table 2, it can be seen that the magnetic tapes ofExamples according to the present invention have the smaller anisotropicmagnetic field distribution than those of Comparative Examples and that,as a result, the block error rate, which is one of the electromagneticconversion characteristics, is small and thus the reliability of themagnetic tapes are good. These results may be due the fact that themagnetic powders comprising the rare earth element (e.g. samarium,neodymium, yttrium, etc.) and the transition metal (e.g. iron, cobalt,etc.) used in the Examples have a high coercive force based on theuniaxial crystalline magnetic anisotropy although their particle shapeis sphere or ellipsoid, the magnetic powders have high saturationmagnetization although their particles are very fine, and furthermorethey have a high filling rate. Furthermore, the iron nitride magneticpowder containing the rare earth element such as samarium, neodymium,yttrium, etc. achieves the uniaxial anisotropy and the higher coerciveforce, and thus attains better block error rates.

According to the above results, it can be seen that the magneticrecording media comprising the magnetic powder according to the presentinvention have apparently better recording properties than thosecomprising the conventional acicular magnetic powder when they have thesame thickness of the magnetic layers, and that such an effect isenhanced as the thickness of the magnetic layer is decreased to 0.3 μmor less. In particular, when the thickness of the magnetic layer is madevery thin, for example, 0.08 μm (Example 20) or 0.10 μm (Example 21),the characteristics hardly deteriorate, and the low block error rate ismaintained. Accordingly, it is understood that the magnetic recordingmedia comprising the rare earth element-iron or rare earth element-ironnitride magnetic powder according to the present invention can exhibittheir properties particularly when the thickness of the magnetic layeris 0.3 μm or less. Such an effect may be based on the specific particleshape and size of the magnetic powder of the present invention.

The magnetic tape which was produced using the magnetic powder of thepresent invention without the orientation in the magnetic field (Example26) and one which was produced using the magnetic powder of the presentinvention with the orientation in the perpendicular direction (Example27) have the lower block error rate than the magnetic tapes in which themagnetic powder particles were oriented in the machine direction(Examples 18-24) Such a result depends on the recording density to bemeasured, the forms of the media, etc. For example, the magnetic tapesof Examples 26 and 27 will exhibit excellent properties in the highrecording density range, or in the form of a disc. At all events, it isapparent that the magnetic powder of the present invention exhibits thebetter properties than the conventional acicular magnetic powderirrespective of the presence or absence of the orientation or thedirection of the magnetic field orientation.

Among the magnetic tapes of Comparative Examples 3-6 comprising theacicular magnetic powder, the magnetic tape having the thickness of themagnetic layer of smaller than 0.3 μm (Comparative Example 5) had theinferior block error rate to one having the thickness of the magneticlayer exceeding 0.3 μm (Comparative Example 4). This is because theacicular magnetic powder has the distribution when it is dispersed inthe magnetic layer and some magnetic powder particles may protrude fromthe surface of the magnetic layer, and thus the surface smoothness ofthe magnetic layer is disturbed. Such a problem is fatal to the acicularmagnetic powder. When the thickness of the magnetic layer of themagnetic tape comprising the acicular magnetic powder is decreased toabout 0.5 μm (Comparative Example 3) or 0.3 μm (Comparative Example 4),the magnetic tapes had decreased block error rate. Those tapes had theinferior magnetic properties to the magnetic tapes of Examples having athickness of the magnetic layer of 0.3 μm or less. Such a result ispeculiar to the longitudinal recording media, since the various types ofdemagnetization occur as the thickness of the magnetic layer increases.

Furthermore, the magnetic tape comprising the magnetic powder consistingof iron and cobalt but containing no rare earth element has the very lowcoercive force and also the greatly worsened block error rate since themagnetic powder has a wide particle size distribution.

(Examples 28-30 and Comparative Example 7) Example 28

The following components for an undercoat layer were kneaded with akneader and dispersed with a sand mill in a residence time of 50minutes. To the mixture, a polyisocyanate (6 parts) was added, stirredand then filtrated to obtain an undercoat paint.

<Components of undercoat paint> parts α-Iron oxide (av. major axislength: 0.14 μm; 65 av. acicular ratio: 7) α-Alumina particles (av.particle size: 0.4 μm) 10 Carbon black (av. particle size: 0.024 μm) 18Carbon black (av. particle size: 0.075 μm) 7 Vinyl chloride-vinylacetate-vinyl alcohol copolymer 16 (SO₃Na groups: 0.7 × 10⁻⁴ eq./g)Polyurethane resin 7 (SO₃Na groups: 1 × 10⁻⁴ eq./g) Oleyl oleate(melting point: <0° C.) 6 n-Butyl stearate (melting point 28° C.) 2Cyclohexanone 200 Methyl ethyl ketone 200<Magnetic Paint Components>

The same magnetic paint as one used in Example 18 was used.

The undercoat paint was applied on a polyamide film having a thicknessof 4 μm as a non-magnetic support to form an undercoat layer having athickness of 2 μm after drying and calendering. On the undercoat layerwhich was still wet, the magnetic paint was applied while applying amagnetic field of 0.3 T along the machine direction so that the magneticlayer had a thickness of 0.20 μm after drying and calendering, and thendried.

Next, on the surface of the non-magnetic support opposite to the surfaceon which the undercoat layer and the magnetic layer were formed, a backcoat paint was applied in the same manner as in Example 18 so that theback coat layer had a thickness of 0.7 μm after drying and calendering,and dried.

The produced magnetic sheet was planish finished with five-stagecalendering (at 70° C. under a linear pressure of 147 kN/m) and aged at60° C., 40% RH for 48 hours with winding the sheet around a sheet core.Then, the sheet was slit at a width of 3.8 mm, and the surface of themagnetic layer of the obtained tape was abraded with a ceramic wheel (arotation speed of +120% and a winding angle of 30 degrees) whiletraveling the tape at a rate of 100 m/min. Thus, a magnetic tape havinga length of 125 m was obtained. The magnetic tape was installed in acartridge and used as a tape for a computer.

Example 29

A magnetic tape for a computer was produced in the same manner as inExample 28 except that 65 parts of a titanium oxide powder (Averageparticle size: 0.08 μm) was used as an inorganic powder in the undercoatpaint in place of 65 parts of α-iron oxide (Average major axis length:0.14 μm, average acicular ratio: 7), and the residence time in thepreparation of the undercoat paint was changed to 60 minutes, and theproduced magnetic sheet was planish polished by the five-stagecalendering (at 800° C. under a linear pressure of 245 kN/m).

Example 30

A magnetic paint was prepared in the same manner as in Example 18 exceptthat the yttrium-containing iron nitride magnetic powder produced inExample 12 (Coercive force: 210.9 kA/m; Saturation magnetization: 15.1μWb/g; Average particle size: 15 nm; Particle shape: sphere orellipsoid) was used as a magnetic powder. Then, a magnetic tape for acomputer was produced in the same manner as in Example 28 except thatthe above-prepared magnetic paint was used.

Comparative Example 7

A magnetic tape for a computer was produced in the same manner as inExample 18 except that a samarium-containing iron acicular magneticpowder (Sm/Fe: 5.1 atomic %; Coercive force: 151.8 kA/m; Saturationmagnetization: 18.3 μWb/g; Particle size: 100 nm; Particle shape:acicular; aspect ratio: 5) was used as a magnetic powder, and theresidence time in the sand mill in the kneading step was changed to 30minuets.

The acicular magnetic powder used in this Comparative Example wasproduced by dispersing acicular goethite (α-FeOOH) particles in theaqueous solution of a samarium salt, depositing samarium hydroxide onthe surfaces of the goethite particles with the solution of an alkali,treating the goethite particles carrying samarium hydroxide with boron,and reducing the intermediate product in the hydrogen gas atmosphere,according to the method for the production of the magnetic powder of thepresent invention explained in the Examples.

The samarium-containing iron acicular magnetic powder used in followingComparative Example 8 was produced in the same method as described aboveexcept that the amounts of the acicular goethite as the startingmaterial and the samarium salt were changed.

The P-V values on the magnetic layer surfaces of the magnetic tapes forthe computer produced in Examples 28-30 and Comparative Example 7 weremeasured using the optical interference type three-dimensional surfaceroughness meter (TOPO-3D).

As the short wavelength output of the magnetic tapes, a peak-to-peakvalue of the output from a playback amplifier at the shortest recordingwavelength of 0.49 μm was measured with an oscilloscope using the sameDDS drive (C 1554 A) as used in the previous Examples to measure theblock error rates. The measured values are expressed as relative valuesto that of the magnetic tape of Comparative Example 7 (100%)

The results are shown in Table 3 together with the properties of themagnetic tapes.

TABLE 3 Thickness Shape of of magnetic Particle magnetic Out- Magneticpower size layer P-V put Ex. No. Powder particles (nm) (μm) (nm) (%) Ex.28 Sm—Fe—Co Sphere or 15 0.2 31 128 ellipsoid Ex. 29 Sm—Fe—Co Sphere or15 0.2 28 133 ellipsoid Ex. 30 Y—N—Fe Sphere or 15 0.2 33 159 ellipsoidC. Ex. 7 Sm—Fe Acicular 100 0.2 81 100 (aspect ratio: 5)

As can be seen from the results of Examples 28-30 summarized in Table 3,the good output is attained even in the case of the short wavelengthrecording, when the magnetic powders of the present invention are used,and the P-V values, which are measured with the optical interferencetype three-dimensional surface roughness meter, are 50 nm or less. Inparticular, the magnetic tape for computer comprising the rare earthelement-iron nitride magnetic powder achieves much better output in theshort wavelength recording than the acicular magnetic powder.

In contrast, in the case of the magnetic tape produced in ComparativeExample 7 which had the same thickness of the magnetic layer and usedthe samarium-containing iron magnetic powder having the same coerciveforce as those in Examples, the magnetic powder particles are easilyagglomerated in the dispersing process, and the magnetic powderparticles penetrate into the undercoat layer in the orientation step sothat the surface of the magnetic layer is roughened. Thus, the surfacesmoothness deteriorates and the output decreases.

According to the present invention, when the thin magnetic layer havinga thickness of 0.3 μm or less is formed, the deterioration of thesurface smoothness of the magnetic layer, which is the problem of theconventional acicular magnetic powder, can be suppressed and the highoutput characteristics can be attained with the recording system usingthe shortest recording wavelength of 1.0 μm or less.

(Examples 31-32 and Comparative Example 8) Example 31

The following components for an undercoat layer were kneaded with akneader and dispersed with a sand mill in a residence time of 60minutes. To the mixture, a polyisocyanate (6 parts) was added, stirredand then filtrated to obtain an undercoat paint.

Separately, the following components (3) for a magnetic paint werekneaded with a kneader and dispersed with a sand mill in a residencetime of 50 minutes. Then, the following components (4) were added to thedispersion, stirred and filtrated to obtain a magnetic paint.

<Components of undercoat paint> parts γ-Iron oxide (av. major axislength: 0.12 μm; 65 aspect ratio: 8; Hc: 23.9 kA/m; σ_(s): 9.4 μWb/g,BET specific surface area: 25 m²/g) α-Alumina particles (av. particlesize: 0.4 μm) 10 Carbon black (av. particle size: 0.024 μm) 25 Vinylchloride-vinyl acetate-vinyl alcohol copolymer 16 (SO₃Na groups: 0.7 ×10⁻⁴ eq./g) Polyurethane resin 7 (SO₃Na groups: 1 × 10⁻⁴ eq./g) Oleyloleate 6 n-Butyl stearate 2 Cyclohexanone 200 Methyl ethyl ketone 200

<Magnetic paint components (3)> parts Samarium-cont. iron-cobaltmagnetic powder 100 according to Example 1 (Coercive force: 12.7 kA/m;Saturation magnetization: 21.6 μWb/g; Av. particle size: 15 nm; Sphereor ellipsoid) Vinyl chloride-hydroxypropyl acrylate copolymer 8 (SO₃Nagroups: 0.7 × 10⁻⁴ eq./g) Polyester polyurethane resin 7 (SO₃Na groups:1.0 × 10⁻⁴ eq./g) α-Alumina (av. particle size: 0.4 μm) 8 Carbon black(av. particle size: 100 nm) 1.5 Myristic acid 1.5 Methyl ethyl ketone133 Toluene 100

<Magnetic paint component (4)> Parts Stearic acid 1.5 Polyisocyanate 4Cyclohexanone 133 Toluene 33

The undercoat paint was applied on a polyamide film (Young's modulus inthe transverse direction (0.3% elongation): 15.7×10⁹ N/m²) as anon-magnetic support to form an undercoat layer having a thickness of 2μm after drying and calendering. On the undercoat layer, the magneticpaint was applied while applying a magnetic field of 0.3 T along themachine direction so that the magnetic layer had a thickness of 0.15 μmafter drying and calendering, and then dried.

Next, on the surface of the non-magnetic support opposite to the surfaceon which the undercoat layer and the magnetic layer were formed, a backcoat paint was applied in the same manner as in Example 18 so that theback coat layer had a thickness of 0.7 μm after drying and calendering,and dried.

The produced magnetic sheet was planish finished with five-stagecalendering (at 80° C. under a linear pressure of 147 kN/m) and aged at60° C., 40% RH for 48 hours with winding the sheet around a sheet core.Then, the sheet was slit at a width of 3.8 mm, and the surface of themagnetic layer of the obtained tape was abraded with a ceramic wheel (arotation speed of +150% and a winding angle of 30 degrees) whiletraveling the tape at a rate of 100 m/min. Thus, a magnetic tape havinga length of 125 m was obtained. The magnetic tape was installed in acartridge and used as a tape for a computer.

Example 32

A magnetic tape for a computer was produced in the same manner as inExample 31 except that the yttrium-containing iron nitride magneticpowder produced in Example 12 (Coercive force: 210.9 kA/m; Saturationmagnetization: 15.1 μWb/g; Av. particle size: 15 nm; Particle shape:sphere or ellipsoid) was used as a magnetic powder, the amounts of thevinyl chloride-hydroxypropyl acrylate copolymer and the polyesterpolyurethane resin were changed to 5 parts and 10 parts, respectively,and a polyamide film (Young's modulus in the transverse direction (0.3%elongation): 13.7×10⁹ N/m²) was used as a non-magnetic support.

Comparative Example 15

A magnetic tape for a computer was produced in the same manner as inExample 31 except that a samarium-containing iron acicular magneticpowder (Sm/Fe: 5.6 atomic %; Coercive force: 165.6 kA/m; Saturationmagnetization: 17.1 μWb/g; Av. particle size: 250 nm; Particle shape:acicular; Aspect ratio: 8) was used as a magnetic powder, the amounts ofthe vinyl chloride-hydroxypropyl acrylate copolymer and the polyesterpolyurethane resin were changed to 15 parts and 2 parts, respectively,and a polyester film (Young's modulus in the transverse direction.(0.3%elongation): 4.4×10⁹ N/m²) was used as a non-magnetic support.

The Young's moduli at 0.3% elongation in the machine direction (Y_(MD))and the transverse direction (Y_(TD)) of the magnetic tapes for thecomputer produced in Examples 31-32 and Comparative Example 8 weremeasured using a tensile tester at 25° C., 60% RH at a stretching rateof 10%/min., and the ratio of Y_(TD) to Y_(MD) was calculated.

The bad head contact of the magnetic tape against the magnetic headleads to the low envelope because of the bias contact, so that thefluctuation of the output increases. Thus, the head contact wasevaluated by measuring the maximum output (A) and the minimum output (B)in one track using the same drive as used in the previous Examples, andcalculating the output ratio: (A−B)/[(A+B)/2].

The result are shown in Table 4 together with the properties of themagnetic tapes.

TABLE 4 Shape of Average magnetic particle Magnetic powder size EnvelopeEx. No. powder particles (nm) Y_(TD)/Y_(MD) (%) Ex. 31 Sm—Fe—Co Sphereor 15 1.41 7 Ellipsoid Ex. 32 Y—N—Fe Sphere or 15 1.38 7 Ellipsoid C.Ex. 8 Sm—Fe Acicular 250 0.87 23 (aspect ratio: 8

From the results of Examples 31-32 summarized in Table 4, it can be seenthat the magnetic tapes of the present invention have better headcontact than the magnetic tape comprising the conventional magneticpowder, when the former magnetic tapes comprise the magnetic powder ofthe present invention, and the ratio of the Young's modulus in thetransverse direction to that in the machine direction (Y_(TD)/Y_(MD)) isin the range between 1.0 and 1.7, that is, the Young's modulus in thetransverse direction is increased. In contrast, since the magnetic tapeof Comparative Example 8 comprises the magnetic powder having theacicular shape and the large particle size, the magnetic powderparticles are easily aligned in the machine direction so that thestrength in the transverse direction is weaker than that in the machinedirection, and thus the sliding contact against the magnetic head islocalized. Accordingly, the head contact is remarkably deteriorated.According to the present invention, the strength in the transversedirection can be increased in relation to the machine direction incomparison with the conventional acicular magnetic powder, and thus thegood head contact can be achieved.

1. A magnetic recording medium comprising a non-magnetic support, atleast one undercoat layer containing an inorganic powder and a binderformed on said support, and a magnetic layer containing a magneticpowder and a binder formed on said at least one undercoat layer, whereinsaid magnetic layer has an average thickness of 0.3 μm or less, and saidmagnetic powder is a magnetic powder of substantially spherical orellipsoidal particles comprising a core and an outer layer, saidparticles comprising a transition metal which comprises iron and a rareearth element which is mainly present in the outer layer of the magneticpowder particles, and having a particle size of 5 to 200 nm, a coerciveforce of 80 to 400 kA/m and a saturation magnetization of 10 to 25μWb/g, wherein said magnetic powder particles are essentially free ofboron, and wherein said magnetic powder has a uniaxial magneticanisotropy one direction of which is a magnetization easy direction. 2.The magnetic recording medium according to claim 1, wherein saidmagnetic powder has an average particle size of 10 to 50 nm.
 3. Themagnetic recording medium according to claim 1, wherein said magneticpowder has a substantially ellipsoidal shape having an average aspectratio of 2 or less.
 4. The magnetic recording medium according to claim1, wherein the content of the rare earth element in said magnetic powderis from 0.2 to 20 atomic % based on the transition metal.
 5. Themagnetic recording medium according to claim 1, wherein said rare earthelement in the magnetic powder is at least one element selected from thegroup consisting of samarium, neodymium and yttrium.
 6. The magneticrecording medium according to claim 1, wherein said transition metal inthe magnetic powder comprises iron and cobalt.
 7. The magnetic recordingmedium according to claim 6, wherein an atomic ratio of cobalt to ironis from 3:97 to 40:70.
 8. The magnetic recording medium according toclaim 1, wherein the core part of the magnetic powder particlescomprises at least one material selected from the group consisting ofmetal iron, iron alloys and iron compounds.
 9. The magnetic recordingmedium according to claim 1, wherein the core part of the magneticpowder particles comprises a mixed phase containing at least one metalcomponent selected from the group consisting of metal iron and ironalloys, and at least one iron compound.
 10. The magnetic recordingmedium according to claim 1, wherein the core part of the magneticpowder particles comprises at least one material selected from the groupconsisting of metal iron, iron alloys and iron compounds, and said ironcompound is iron nitride, or iron nitride a part of iron atoms of whichare replaced with at least one transition metal element.
 11. Themagnetic recording medium according to claim 1, wherein the core part ofthe magnetic powder particles comprises at least one material selectedfrom the group consisting of metal iron, iron alloys and iron compounds,and said iron compound is Fe₁₆N₂, or Fe₁₆N₂ a part of iron atoms ofwhich are replaced with at least one transition metal element.
 12. Themagnetic recording medium according to claim 1, wherein the core part ofthe magnetic powder particles comprises at least one iron compoundselected from the group consisting of Fe₁₆N₂, and Fe₁₆N₂ a part of ironatoms of which are replaced with at least one transition metal element,and said iron comopund contains 1.0 to 20 atomic % of nitrogen based oniron.
 13. The magnetic recording medium according to claim 1, whereinsaid magnetic layer has magnetization-easy-directions along thelongitudinal direction of said magnetic layer, and said magnetic layerhas a coercive force of 80 to 400 kA/m, a squareness of 0.6 to 0.9, anda saturation magnetization of 0.1 to 0.5 T in the longitudinaldirection.
 14. The magnetic recording medium according to claim 1,wherein said magnetic layer has magnetization-easy-directions along thedirection perpendicular to the plane of said magnetic layer, and saidmagnetic layer has a coercive force of 60 to 320 kA/m, a squareness of0.5 to 0.8, and a saturation magnetization of 0.1 to 0.5 T in theperpendicular direction.
 15. The magnetic recording medium according toclaim 1, wherein said magnetic layer has magnetization-easy-directionsrandomly distributed in the plane of said magnetic layer, and saidmagnetic layer has a coercive force of 45 to 320 kA/m, a squareness of0.4 to 0.7, and a saturation magnetization of 0.1 to 0.5 T in anydirection in the plane of the magnetic layer and in the directionperpendicular to the magnetic layer.
 16. A magnetic powder consisting ofsubstantially spherical or ellipsoidal particles comprising a core andan outer layer, said particles comprising a transition metal whichcomprises iron and a rare earth element which is mainly present in theouter layer of the magnetic powder particles, and having a particle sizeof 5 to 200 nm, a coercive force of 80 to 400 kA/m and a saturationmagnetization of 10 to 25 μWb/g, wherein said magnetic powder particlesare essentially free of boron.
 17. The magnetic powder according toclaim 16, wherein the content of said rare earth element is from 0.2 to20 atomic % based on the transition metal.
 18. The magnetic powderaccording to claim 16, wherein said rare earth element is at least oneelement selected from the group consisting of samarium, neodymium andyttrium.
 19. The magnetic powder according to claim 16, wherein saidtransition metal comprises iron and cobalt.
 20. The magnetic powderaccording to claim 19, wherein an atomic ratio of cobalt to iron is from3:97 to 40:70.
 21. The magnetic powder according to claim 19, whereinsaid transition metal further comprises nickel.
 22. The magnetic powderaccording to claim 16, wherein the core part of the magnetic powderparticles comprises at least one material selected from the groupconsisting of metal iron, iron alloys and iron compounds.
 23. Themagnetic powder according to claim 16, wherein the core part of themagnetic powder particles comprises a mixed phase containing at leastone metal component selected from the group consisting of metal iron andiron alloys, and at least one iron compound.
 24. The magnetic powderaccording to claim 16, wherein the core part of the magnetic powderparticles comprises at least one material selected from the groupconsisting of metal iron, iron alloys and iron compounds, and said ironcompound is iron nitride, or iron nitride a part of iron atoms of whichare replaced with at least one transition metal element.
 25. Themagnetic powder according to claim 16, wherein the core part of themagnetic powder particles comprises at least one material selected fromthe group consisting of metal iron, iron alloys and iron compounds, andsaid iron compound is Fe₁₆N₂, or Fe₁₆N₂ a part of iron atoms of whichare replaced with at least one transition metal element.
 26. Themagnetic powder according to claim 25, which is a substantiallyspherical or ellipsoidal rare earth-iron nitride magnetic powdercomprising a rare earth element, iron and nitrogen, and containing atleast a Fe₁₆N₂ phase, wherein the contents of the rare earth element andnitrogen are 0.2 to 20 atomic % and 1.0 to 20 atomic %, respectivelybased on iron, and said magnetic powder has an average particle size of10 to 50 nm and an average aspect ratio of 2 or less.
 27. The magneticpowder according to claim 26, wherein said rare earth element is atleast one element selected from group consisting of samarium, neodymiumand yttrium.
 28. The magnetic powder according to claim 25, which has acoercive force of 119.4 to 318.5 kA/m and a saturation magnetization of10 to 20 μWb/g.
 29. The magnetic powder according to claim 25, which hasa BET specific surface area of 40 to 100 m²/g.
 30. A method forproducing the magnetic powder according to claim 16 which contains arare earth element and a transition metal comprising iron, said methodcomprising the steps of: dispersing substantially spherical orellipsoidal particles of magnetite or cobalt ferrite in an aqueoussolution containing at least a rare earth element ion to form adispersion, adding an aqueous solution of an alkaline material to saiddispersion in a molar amount sufficient for converting said rare earthelement ion to a hydroxide to form a surface layer of the hydroxide onthe particles of magnetite or cobalt ferrite, recovering the particles,drying and reducing the particles by heating to obtain substantiallyspherical or ellipsoidal particles having a particle size of 5 to 200nm, a coercive force of 80 to 400 kA/m and a saturation magnetization of10 to 25 μWb/g.
 31. A method for producing the rare earth element-ironnitride magnetic powder according to claim 24 comprising the steps of:supplying an iron hydroxide or an iron oxide in the form of particles asa raw material, depositing a rare earth element on the surface of theparticles of the iron hydroxide or the iron oxide, reducing saidparticles carrying the rare earth element by heating, and nitriding saidparticles at a temperature lower than a temperature in the reducingstep.